re: the coverage fix moved 29 anchors, 22 of which carried another resource's geometry

Whole-disc comparison at XBG7_COVER_SLACK=4 vs =1: 29 resources changed anchor,
3 stopped decoding, 6207 unchanged. Of the moved ones present in several
containers, 0 matched the sibling consensus before and 9 after -- it moved them
onto the block their own copies agree on and none away. 22 of 29 were carrying
ANOTHER resource's geometry under their own name, including a shift chain in
Stage_S28 where n054_bdy_l held n056_bdy_l's mesh and n056_bdy_l held
n055_bdy_l's. Confidently-wrong-under-the-right-name is invisible to every
count-based metric; the coverage invariant is what separated them.

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

docs/re/structures/xbg7-mesh.md

@@ -1097,6 +1097,33 @@ that carry it) and `t901_e01_D` is 58×59×3013 (a mast). And `f101_bdy_01` flag
 at 6× while being **capture-verified exact** in the truth table — a useful
 reminder that a bulky hull is not a bug.
 
+### What the exact-coverage fix actually reached
+
+The fix was justified on one resource (`e106_bdy_03`) and one render. Comparing
+whole-disc decodes at `XBG7_COVER_SLACK=4` and `=1` shows what else moved:
+
+- **29 resources changed anchor**, 3 stopped decoding, 6 207 unchanged.
+- Of the moved ones that appear in several containers, **0 matched the sibling
+  consensus before the fix and 9 match it after** — the fix moved them onto the
+  block their own copies agree on, and moved none away from it.
+- **22 of the 29 were carrying another resource's geometry** under their own
+  name.
+
+The clearest case is a **shift chain** in `Stage_S28`: `n054_bdy_l` was decoding
+`n056_bdy_l`'s geometry, and `n056_bdy_l` was decoding `n055_bdy_l`'s — each
+resource landing on a neighbour's block, all of them plausible-looking meshes of
+the right vertex count. `n054_bdy_l` decoded `(396, 315, 328)` in four containers
+where its own copies agree on `(439, 284, 270)`.
+
+That is the failure mode that matters for a reimplementation: not a missing
+decode, but a **confidently wrong one under the right name** — and no
+count-based metric can see it, because every count is correct. It took the
+coverage invariant (real blocks reach their pool's last vertex) to separate them.
+
+(Seven of the 29 carried geometry no resource claims. And the `ptc_pack.xpr`
+`eff_*` entries in the list are small effect quads that all share one span, so
+their "owner" attribution is weak — noted rather than counted.)
+
 ### The containment screen, and a correction: the assembler is NOT at fault
 
 `examples/envelope_screen.rs` is the metric the eye actually used: assemble a