fix(xbg7): degenerate index runs 582 -> 1 (grouped path + prefer a clean candidate)

Two follow-ups to the pad-scoring fix, both driven by the same invariant (a
correctly located index run has no degenerate triangles):

- anchor_grouped_meshes picked its pad by first-match too; scoring the pivot run
  the same way cleared every remaining ptc_pack composite (f102/f104/e107).
- anchor_pool_mesh now prefers a degenerate-free candidate over an earlier dirty
  one. examples/better_home.rs showed the last two resources each had exactly one
  degenerate-free, pool-covering block, sitting later in file order than the
  lookalike we took. First-match order is kept for every clean hit, and a dirty
  block is still used if nothing clean exists, so coverage cannot regress.

  degenerate index runs, disc-wide:      582 -> 11 -> 1
  captured index runs identical:         93/93 (unchanged)
  resources decoded / misses:            6 209 / 85 (unchanged)
  index runs changed / anchors moved:    590 / 10 (_rou_f402_dead x8, e201_bdy_03_m x2)

Cross-container minority decodes 89 -> 96, and that is progress: all seven new
rows are _rou_f402_dead, which now has a majority (32x25x8) for the first time, so
its seven wrong copies are named instead of hidden behind "no majority".

The last dirty run (_rou_f402_dead in Stage_S09) is blocked by distinct assignment
— its clean block is claimed by e_rou_f003_Near, both 24-vertex bounding boxes. A
winding-floor escalation for that case was written, measured to fire for nothing,
and reverted; the reasoning is kept as a comment.

Regression threshold tightened to 1. Suite green with --include-ignored apart from
the pre-existing known-failing cross-container consistency test.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NptfmpjdpNCKEez6d2xvA9

docs/re/INDEX.md

@@ -19,7 +19,7 @@ Promote to a prose `structures/…md` file when a format needs behavioural notes
 | LSTA sprite list | ✅ | `sylpheed-formats/src/lsta.rs` | A display list of inline elements: **T8aD sprites and `PRMD` primitives**. The `count` at `0x04` is **exact and counts both** — `count == T8aD + PRMD` for **64/64** lists on the disc, which retires the old "a few entries disagree" note (it compared sprites against a total including primitives). **All 1 281 sprite frames decode** after the T8aD rectangle-list fix |
 | IXUD subtitle | 🟡/✅ | `sylpheed-formats/src/ixud.rs` + [movie link](movie-subtitle-link.md) | timed cues. **The movie↔subtitle↔voice link is solved — statically**, from the movie config record in `tables.pak` (schema `0x067025b9`), not from the running game as this row previously assumed: [101 movies mapped](captures/movie-subtitle-voice-map.csv), 94 with subtitles, 83 with voice, 21 with a telop overlay. 93 of 94 subtitle refs resolve in the language paks; **`SUBTITLE_S12B.tbl` is missing from all six languages** — a dangling reference on the disc. Naming is `SUBTITLE_<base>.tbl` / `VOICE_<base>` with six documented exceptions. The record's ~104 **script ids** are ❔ — positional pairing drifts by three because the IDXD pool dedupes repeated values |
 | Fonts (ttf/otf/ttc) | ✅ | `sylpheed-formats/src/font.rs` | standard OpenType, parsed via ttf-parser |
-| XBG7 mesh | ✅/🟡 | `sylpheed-formats/src/mesh.rs` + `tests/mesh_disc.rs` ([xbg7](structures/xbg7-mesh.md)) | **6 294 resources, 6 209 decode (98.7 %), 82 searched-and-missed** (2026-08-12, up from 5 480 / 87.1 %). Five evidence-driven fixes got there: **distinct anchor assignment** (no two resources may claim one buffer — proved by a capture showing the container holds both mirrored `e106` hull halves), the connectivity cap replaced by a **winding-consistency gate at 0.70**, **structural requirements on pre-pivot sub-meshes** (index range, then exact pool coverage), and **filtering after the assignment** so a subset query cannot differ from the full decode. Validated against a runtime capture that names the file offset of every buffer the engine drew: **46/46 drawn buffers claimed, 45 anchored exactly**. **No real mesh now decodes differently in different containers** — all 89 remaining cross-container disagreements are interchangeable 24-vertex bounding boxes, which no anchoring rule can pin (monotone order re-tested and refuted). Remaining misses attribute to the degeneracy/extent gate (42), winding (31) and coverage (9); the first was probed and its "obvious" fix refuted. Every decoded sub-mesh covers its own vertex pool. **The `[index buffer][vertex buffer]` layout is now runtime-verified** (2026-08-13): with the F10 capture extended to log each draw's index buffer, all **42** drawn `Stage_S02` buffers match our decoded index count exactly, all 42 have their index union cover the pool exactly, and the 30 single-block cases all sit at `pad ≤ 3` — so `e106_eng_02_l`'s old rejection was the connectivity gate, not a misplaced index buffer. The `indices=` mystery was the capture keeping only the **first of several index batches** per buffer. **And comparing index VALUES found the biggest silent defect yet**: the anchor took the first `pad` that validated, so a block whose index data sits at pad 2 was read **one element late** — 76/93 captured runs matched, all 17 differences a one-element shift. Scoring pads by degenerate triangles + winding fixes it: **93/93** captured runs now match byte for byte, disc-wide degenerate runs **579 → 16**, **575 of 8 850** sub-meshes re-wired, with resources decoded, vertex anchors and cross-container consistency all unchanged |
+| XBG7 mesh | ✅/🟡 | `sylpheed-formats/src/mesh.rs` + `tests/mesh_disc.rs` ([xbg7](structures/xbg7-mesh.md)) | **6 294 resources, 6 209 decode (98.7 %), 82 searched-and-missed** (2026-08-12, up from 5 480 / 87.1 %). Five evidence-driven fixes got there: **distinct anchor assignment** (no two resources may claim one buffer — proved by a capture showing the container holds both mirrored `e106` hull halves), the connectivity cap replaced by a **winding-consistency gate at 0.70**, **structural requirements on pre-pivot sub-meshes** (index range, then exact pool coverage), and **filtering after the assignment** so a subset query cannot differ from the full decode. Validated against a runtime capture that names the file offset of every buffer the engine drew: **46/46 drawn buffers claimed, 45 anchored exactly**. **No real mesh now decodes differently in different containers** — all 89 remaining cross-container disagreements are interchangeable 24-vertex bounding boxes, which no anchoring rule can pin (monotone order re-tested and refuted). Remaining misses attribute to the degeneracy/extent gate (42), winding (31) and coverage (9); the first was probed and its "obvious" fix refuted. Every decoded sub-mesh covers its own vertex pool. **The `[index buffer][vertex buffer]` layout is now runtime-verified** (2026-08-13): with the F10 capture extended to log each draw's index buffer, all **42** drawn `Stage_S02` buffers match our decoded index count exactly, all 42 have their index union cover the pool exactly, and the 30 single-block cases all sit at `pad ≤ 3` — so `e106_eng_02_l`'s old rejection was the connectivity gate, not a misplaced index buffer. The `indices=` mystery was the capture keeping only the **first of several index batches** per buffer. **And comparing index VALUES found the biggest silent defect yet**: the anchor took the first `pad` that validated, so a block whose index data sits at pad 2 was read **one element late** — 76/93 captured runs matched, all 17 differences a one-element shift. Scoring pads by degenerate triangles + winding fixes it: **93/93** captured runs now match byte for byte, disc-wide degenerate runs **582 → 1** (the grouped path had the same bug; and two resources were anchored on a degenerate lookalike earlier in file order), **590 of 8 850** sub-meshes re-wired with 10 vertex anchors moved, resources decoded unchanged at 6 209. Cross-container minority decodes 89 → 96 — *because* the decoder improved: `_rou_f402_dead` now has a majority (32×25×8) so its seven wrong copies are named instead of hidden. One dirty run remains, blocked by distinct assignment on a 24-vertex box |
 | Capital-ship part placement | ✅ | `sylpheed-formats/src/ship.rs` (static) + [runtime capture](ship-placement-runtime-capture.md) | Placement is **sound** (hull static-exact against the `e106` capture; cross-id mounting genuinely narrow, 2 pairs across 335 ships). The XBG7 mis-decode this row used to blame for "ships assemble wrong" — a shared turret ~100× too large in some containers — is **fixed** (2026-08-12, the exact-coverage requirement): `e303_wep_01` now decodes 49×23×42 everywhere and places at ±179 on the `e106` hull, and no real mesh disagrees across containers. A composite-node audit confirmed the assembler itself never applied a bad scale (all nodes scale 1.0, orthonormal). Still open: `static_assembly_matches_runtime_capture` walks capture parts only, so **extra** static placements cannot fail it |
 | Weapon fields defaulted on disc | ✅ | [runtime struct](structures/weapon-struct-runtime.md) · [DATA SHEET route](weapon-datasheet-runtime.md) | **Solved.** Canary maps guest RAM into `/dev/shm`, so the parsed `Weapon`/`Shell` objects are readable live; their layout is solved against disc ground truth (zero contradictions over 100+ records). All 126 weapons, exact numbers, no story progress needed — [4 393 values](captures/weapon-runtime-fields.csv) the disc does not carry. Supersedes the letter-bucket limit of the DATA SHEET route, which now serves as the independent cross-check |
 | Unit (craft/vessel) fields defaulted on disc | ✅/🟡 | [runtime struct](structures/unit-struct-runtime.md) | The parsed `unit\UN_*.tbl` definition object, vtable `0x820af844`, ≥`0x380` bytes, one per unit — **discovered, not assumed** (`unit_discover.py`), and distinguished from the spawned-entity class `0x820af030` by being one-per-ID and byte-constant within a run. Across runs only pointer words move — `--crosscheck` proves **no reported field offset is run-dependent** (two words, `+0x2c8`/`+0x2d0`, are stage-dependent and remain unidentified). 27 fields ✅ (21 units, 7 runs); the `Maneuver` block is **schema declaration order, 4 bytes/field, base `0x9c` with a two-slot gap after `AA_Roll_Min`** (29 anchors, 0 conflicts), which also pins 5 fields *no* disc record ever values. Angles are **radians at runtime, degrees on disc**. **Re-derived independently 2026-08-13 from the loader's own key strings** (`sub_82341A20`; the field name for each store is a string in the image): **159 fields**, agreeing with this solver on **25 of 25 shared offsets**, verified at **406 values matching the disc and 0 disagreeing** over 11 live objects spanning UNIT and VESSEL — landed as `data/unit_definition_layout.txt` + `sylpheed_formats::unit_layout` + a no-emulator test, with **121 defaulted fields** read out ([live-unit-definitions](live-unit-definitions.md)). Unlike weapons, unit definitions are instantiated **per stage**, so coverage (21/110) grows by visiting missions — but a defaulted field is **not** a global constant: `Size_Y` provably inherits `Size_X` (7 independent units, 6 distinct values), and three more sibling rules are recorded ❔, recovering 65 values in units never visited — [values](captures/unit-runtime-fields.csv) |

docs/re/structures/xbg7-mesh.md

@@ -1458,3 +1458,53 @@ panel lines, bridge tower and funnels. Same 31 991 triangles, same 4 placements,
 same bounds — only the wiring differs. Third time this project has learned it:
 **render the output**; a metric that cannot see a 1 600-unit slab could not see
 this either.
+
+### ✅ Following it through: degenerate index runs 582 → 1 (2026-08-13)
+
+The pad-scoring fix left 16 dirty sub-meshes. Two follow-ups cleared all but one.
+
+**1. The grouped path had the same first-match bug.** `anchor_grouped_meshes`
+picks `ib0 = vb0 − span − pad` the same way, so it got the same scoring (pivot
+run's degenerate count, then winding). That cleared every remaining `ptc_pack`
+composite — `f102_break.dat` (161 degenerate triangles), `f104_break.dat` (3
+sub-meshes) and `e107_break.dat` — 16 → 11.
+
+**2. Prefer a degenerate-free candidate over an earlier dirty one.** The last two
+resources were anchored on a *lookalike earlier in file order*:
+`examples/better_home.rs` scores every candidate `(start, pad)` for a resource and
+showed **exactly one** degenerate-free, pool-covering block for `e201_bdy_03_m`
+(`0x316383C`, ours was `0x248D054` with 4 degenerate triangles) and one for
+`_rou_f402_dead`. So `anchor_pool_mesh` now keeps first-match order for every
+clean hit and only searches on when the accepted block is provably mis-fitted,
+falling back to the dirty block if nothing clean exists (coverage can never
+regress). 11 → 1.
+
+| | first-match | + pad scoring | + these two |
+|---|---|---|---|
+| decoded sub-meshes with a degenerate index run (disc-wide) | 582 | 11 | **1** |
+| captured index runs identical (`Stage_S02`) | 76/93 | 93/93 | **93/93** |
+| resources decoded / misses | 6 209 / 85 | 6 209 / 85 | **6 209 / 85** |
+| sub-meshes whose index run changed | — | 580 | 590 |
+| of those, vertex anchor moved | — | 0 | **10** (`_rou_f402_dead` ×8, `e201_bdy_03_m` ×2) |
+
+**The consistency screen went 89 → 96 minority decodes, and that is progress.**
+All seven new rows are `_rou_f402_dead`: eight containers now agree on a
+**32 × 25 × 8** box, which gives the resource a *majority for the first time*, so
+the seven copies that still land elsewhere (`160×160×78`, `519×100000×519`,
+`856×463×803`, `779×5769×5769`) are finally **named** instead of hiding behind "no
+majority". Textbook case of consistency being the weaker witness: the number rose
+because the decoder got better.
+
+**The one remaining dirty run** is `_rou_f402_dead` in `Stage_S09`. Its
+degenerate-free block (`0x17AE620`, validates at pad 0) is **claimed by
+`e_rou_f003_Near`** — so distinct assignment blocks it, not a gate. Both are
+24-vertex bounding boxes: the identity class that needs descriptor-level data, not
+another heuristic. A winding-floor escalation for this case was written, measured
+to fire for **nothing** on the disc, and reverted (the comment survives in
+`anchor_pool_mesh`).
+
+**Next target, with the caveat that killed the last attempt:** six containers
+decode `_rou_f402_dead` to a *clean but wrong* block, so several degenerate-free
+candidates exist and file order picks badly. The majority span would separate
+them — but "flag, don't silently rewrite geometry on a vote" still stands, so this
+needs the box-identity data, or a capture of a stage that draws it.