re: the capture names file offsets -- the twin collapse is an anchoring error

Locating each drawn buffer's positions inside Stage_S01.xpr shows vbase - offset
is one constant (0x1A94FFF4, same in two runs), so a capture gives ground truth
at file-offset granularity. Read against our anchor scan it is a defect list:
full/_m resources starting at their own _l buffer, eight drawn buffers claimed by
nobody, and the bdy_01_l/bdy_02_l twins sharing one buffer while the container
carries both halves (0x3b3ee8 and its exact X-mirror at 0x3c55d8).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/re/structures/xbg7-mesh.md

@@ -582,4 +582,66 @@ resource by `vcount` plus decoded positions. The same match yields, for each
 part, the `vbase` the engine used — so two resources that our decoder gives the
 same geometry can be checked directly: different `vbase` in the capture ⇒ our
 shared decode is wrong. That is the per-part oracle any future anchor work should
-be validated against, and it needs no new capture run.
+be validated against, and it needs no new capture run.
+### ✅ It was run — and the capture gives file-offset ground truth
+
+`examples/shared_vbase_check.rs` does the per-part check above, and then goes one
+step further than planned. Three results, in order of strength.
+
+**1. A draw's `vbase` *is* the container file offset plus a constant.** Vertex
+POSITION is `f32×3` big-endian at vertex offset 0, so a draw's dumped positions
+are a value pattern that can be searched for in the `.xpr` itself. Doing that for
+every draw in `xenia_ship_capture_01/02.log` and histogramming `vbase − offset`:
+
+```
+xenia_ship_capture_01.log: 11 distinct vbases located, 208 not in this container
+   vbase - offset = 0x1A94FFF4  ×8      ← same constant in log 02
+```
+
+The 208 "not in this container" are draws whose geometry lives in `Common.xpr`,
+a weapon pack or a backdrop — expected. The eight that do belong to `Stage_S01`
+share **one** constant, and the *same* constant in a second run, so the container
+is uploaded contiguously and **a capture names the exact file offset of every
+buffer the engine drew**. Log 03 loaded the container at a different address, so
+the constant is per-run, not baked.
+
+**2. Read against our anchor scan, that is a defect list**
+([`captures/stage-s01-capture-truth-offsets.txt`](../captures/stage-s01-capture-truth-offsets.txt)):
+
+| file offset | drawn vcount | claimed by our decode |
+|---|---|---|
+| `0x3b3ee8` | 119 | `e106_bdy_01_l`(119), `e106_bdy_02_l`(119) |
+| `0x3c55d8` | 119 | — **nobody** |
+| `0x3dd2c4` | 146 | `e106_bdy_03`(815), `e106_bdy_03_l`(146) |
+| `0x40763c` | 179 | `e106_bdy_04_l`(179) ✅ |
+| `0x40e418` | 51 | `e106_brg_01`(202), `e106_brg_01_m`(92) |
+| `0x444ccc` | 58 | `e106_eng_01_l`(58) ✅ |
+| `0x45705c` | 82 | `e106_wep_02_01_l`(82), `e106_wep_02_01_m`(294) |
+| `0x38788` `0x4b8b8` `0xb6574` `0xdbbac` `0x133da0` `0x162840` `0x44a32c` | 181, 93, 41, 77, 76, 60, 44 | — **nobody** |
+
+Two resources land exactly (`bdy_04_l`, `eng_01_l`). Everywhere else a **full or
+`_m` resource starts at the offset of its own `_l` buffer** — `bdy_03` decodes
+815 vertices beginning where the engine's 146-vertex LOD begins — and eight
+drawn buffers are claimed by no resource at all. This is the anchor scan taking
+the first candidate that validates, seen directly rather than inferred.
+
+**3. The twin pair is an anchoring error, and the mirror is in the data.** For
+`e106_bdy_01_l` ≡ `e106_bdy_02_l` the capture shows **two** 119-vertex buffers
+per run, `0x3b3ee8` and `0x3c55d8`; the first is byte-for-byte what we decode,
+and the second is its **exact X-reflection** (every dumped position matches ours
+with `x` negated). So the container carries both halves as separate baked
+geometry, the engine draws each from its own buffer, and our decoder returning
+one buffer for both names is the defect — which `correlate`'s mirror flag and
+`ship::apply_twin_mirrors` have been compensating for downstream all along.
+
+That settles the question this section opened with, for this pair: **not
+legitimate reuse.** It also pins what the withdrawn neighbourhood-anchor fix
+could not: `0x3b3ee8` stays with whichever twin we already decode there, and the
+other twin must move to `0x3c55d8`. The invariant is checkable without a capture
+— *mirrored twins must decode to X-reflected buffers, never identical ones*.
+
+Not settled: `e106_brg_01_b_02` ≡ `e106_brg_01_l` (51 verts). A second 51-vertex
+`vbase` exists in the logs but is **not** from this container, and the container
+holds three near-identical 51-vertex runs, so the pair has no oracle yet.
+`n006_01A` ≡ `n006_01B` shows a single `vbase` in all three logs — consistent
+with real reuse, but equally with only one of the two being on screen.