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/src/ship.rs

@@ -640,14 +640,19 @@ mod tests {
         let count = |res: &str| placed.iter().filter(|p| p.resource == res).count();
         assert_eq!(count("e106_eng_01"), 2, "both engine nacelles placed");
         assert_eq!(count("e303_wep_01"), 2, "both shared turrets placed");
-        // The mirrored starboard hull reflects (det < 0), the port one doesn't.
+        // NEITHER hull reflects: the twins' geometry is mirrored on the disc,
+        // so both placements are proper rotations. This flipped on 2026-08-12 —
+        // while both twins decoded to one buffer, `apply_twin_mirrors` had to
+        // synthesise the reflection here; a runtime capture showed the container
+        // holds both halves, and distinct anchor assignment now hands each twin
+        // its own (see docs/re/structures/xbg7-mesh.md).
         let det = |m: &[[f32; 3]; 3]| {
             m[0][0] * (m[1][1] * m[2][2] - m[1][2] * m[2][1])
                 - m[0][1] * (m[1][0] * m[2][2] - m[1][2] * m[2][0])
                 + m[0][2] * (m[1][0] * m[2][1] - m[1][1] * m[2][0])
         };
         let one = |res: &str| placed.iter().find(|p| p.resource == res).unwrap();
-        assert!(det(&one("e106_bdy_02").m) < 0.0, "starboard hull mirrored");
+        assert!(det(&one("e106_bdy_02").m) > 0.0, "starboard hull plain (mirror is in the data)");
         assert!(det(&one("e106_bdy_01").m) > 0.0, "port hull plain");
     }