fix(mesh): distinct anchor assignment -- no two resources may claim one buffer

Selection was per-resource and greedy, so two resources could take one vertex
buffer while a valid one sat unused. A runtime capture proved that wrong for the
mirrored e106 hull twins: the container holds both halves and the engine draws
each from its own address. Now the first claimant keeps a buffer and later
resources re-anchor past everything already claimed (coverage can never regress;
grouped-pool models untouched).

Against the 46 capture-named Stage_S02 buffers: exact anchors 29 -> 40, unclaimed
12 -> 4. Disc-wide: 5480 resources decoded (unchanged), cross-container
inconsistency 125 -> 46.

The twins' mirror therefore lives in the DATA, not in the placement matrix: the
embedded e106_bdy_02 row and the two assertions encoding the old convention are
updated, each with the reason recorded. Full suite green incl. disc/ISO gates.

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

crates/sylpheed-formats/data/ship_placements.txt

@@ -12,7 +12,12 @@
 
 ship e106 ref=e106_bdy_04
   e106_bdy_01 1 0 0 0 1 0 0 0 1 -263.9948 -150.43242 1164.8667
-  e106_bdy_02 -1 0 0 0 1 0 0 0 1 264.0088 -150.41339 1164.8651
+# 2026-08-12: this rotation was diag(-1,1,1) - the X-flip `correlate` bakes in
+# when a draw's buffer is the mirror of the decoded one. With distinct anchor
+# assignment the twins now decode to their OWN buffers (the container carries
+# both halves), so the mirror lives in the data and the placement is plain
+# identity. Re-emitting from the capture agrees: `1 0 0 0 1 0 0 0 1 264.04343`.
+  e106_bdy_02 1 0 0 0 1 0 0 0 1 264.0088 -150.41339 1164.8651
   e106_bdy_03 1 0 0 0 1 0 0 0 1 0.0029247368 -34.517372 1075.878
   e106_bdy_04 1 0 0 0 1 0 0 0 1 0 0 0
   e106_brg_01 1 0 0 0 1 0 0 0 1 -0.005471501 153.31795 -164.03748