re: a detector for silent XBG7 mis-decodes -- 125 of 681 shared resources fail it

Following the e106 slab to its cause. e303_wep_01 decodes from 14 containers: in
11 it is a 49x23x42 turret with organic vertices, in Stage_S02/S08/S26 the same
resource -- identical 172 verts and 330 indices -- decodes to 1600x2100x4800 of
axis-aligned box corners with (0,0,1) normals. The anchor scan found a different
buffer that happens to share both counts, so every size-based check passes. That
is the "declined for location, not format" risk succeeding wrongly instead of
declining.

The detector needs no ground truth: a resource shared by several containers must
decode to the same bounds. 681 resources appear in >=2 containers and 125 decode
to different bounds while reporting identical vertex/triangle counts -- a lower
bound, since a resource wrong everywhere is invisible to it. _rou_f401 is
62x25x10 in 16 containers and 4738x3147x4738 in 2; _rou_e011_wep05 gives four
different spans across 8.

Majority vote would resolve 104 of the 125 (14 are exact 50/50 splits) and agrees
with ground truth in the one case that has a render and a runtime capture behind
it. Recorded as a heuristic, not applied: for _rou_e302_base_break the majority
is the LARGER span and nothing yet says which is right. Flag, do not silently
rewrite geometry on a vote.

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

docs/re/structures/xbg7-mesh.md

@@ -289,3 +289,52 @@ code already tests. Anyone attacking this should not spend time on the
 descriptor: 229 of the declined resources even carry the *most* common
 `0x00010001` with under 1 KB of data, i.e. they are small meshes the scan has too
 little signal to anchor, not exotic formats.
+
+## Silent mis-decodes: a detector, and how many there are (2026-08-11)
+
+The [declined set](#the-declined-set-measured-2026-08-11) is the *honest* failure
+mode — 814 resources the decoder refuses. This is the other kind: geometry that
+decodes without complaint and is wrong.
+
+### The case that exposed it
+
+`e303_wep_01` decodes from fourteen containers. In eleven it is a **49 × 23 × 42**
+turret with organic vertices (`24.55, 0.00, 4.46` …). In `Stage_S02`, `S08` and
+`S26` the *same* resource — **identical 172 vertices and 330 indices** — decodes
+to **1600 × 2100 × 4800** of axis-aligned box corners:
+
+```
+Stage_S01   [   24.55    0.00    4.46]  [  24.55   9.84   2.91]   normals varied
+Stage_S02   [   42.00 -900.00 2400.00]  [-600.00 -500.00 -500.00]  normals (0,0,1)
+            [ -600.00 -500.00 -950.00]  [-600.00 -950.00 -950.00]  ← box face corners
+```
+
+The anchor scan located a **different buffer that happens to share the vertex and
+index counts**, so every size-based check it makes passes. This is exactly the
+"declined only for *location*, not format" risk the module notes describe — except
+here it does not decline, it succeeds wrongly.
+
+### The detector: cross-container bounds consistency
+
+A resource shared by several containers must decode to the same bounds. That
+needs no ground truth, and it measures the problem:
+
+- **681 resources appear in ≥2 containers.**
+- **125 of them decode to different bounds while reporting identical vertex and
+  triangle counts** — a lower bound on silent mis-decodes (a resource wrong in
+  *every* container is invisible to this test).
+
+Examples: `_rou_f401` decodes as `62×25×10` in 16 containers and `4738×3147×4738`
+in 2; `_rou_e011_wep05` produces **four** different spans across 8 containers.
+
+### Repair candidate, and its limits
+
+Taking the **majority span** across containers resolves **104 of the 125**; 14 are
+exact 50/50 splits that a vote cannot decide. It agrees with the ground truth in
+the one case that has independent evidence — `e303_wep_01`, where the 11-container
+majority is the turret the render and the runtime capture both support.
+
+🟡 **It is a heuristic and is otherwise unvalidated.** For
+`_rou_e302_base_break` the majority is the *larger* span (`685×1206×1444`, 8 of
+15) and nothing yet says which is right. Use the detector to flag; do not silently
+rewrite geometry on a vote.