test: lock in the twin invariant -- 0 unrelated pairs disc-wide, 1 known collapse

twin_mirror_audit applies the capture-derived rule to all 166 containers: of 34
equal-count twin pairs, 18 are exact X-mirrors, 15 related another way, 1
identical, 0 unrelated. Two calibration fixes were needed first (authored halves
need a tolerance, and a mirrored pair may be stored in another vertex order).
The one collapse, n206_01/_02, is a grouped-pool pair -- the path distinct
assignment excludes -- so it names the next target. Added a disc-gated
regression test; refreshed the stale ignore message on the consistency test.

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

crates/sylpheed-formats/tests/mesh_consistency_disc.rs

@@ -53,7 +53,7 @@ fn span(m: &Xbg7Model) -> Option<[i64; 3]> {
 }
 
 #[test]
-#[ignore = "known-failing: 125 of 681 shared resources decode inconsistently. A neighbourhood anchor took this to 51 but regressed the e106 twin-mirror decision and was withdrawn — see docs/re/structures/xbg7-mesh.md"]
+#[ignore = "known-failing: 62 of 714 shared resources decode inconsistently (was 125 of 681; distinct anchor assignment + the 0.42 connectivity cap fixed the rest). Note this metric is the WEAKER witness — a systematic mis-anchor is consistent — see docs/re/structures/xbg7-mesh.md"]
 fn shared_resources_decode_identically_in_every_container() {
     let Some(root) = disc_root() else {
         eprintln!("SKIP: extracted disc not found (set SYLPHEED_DISC to enable)");
@@ -102,3 +102,45 @@ fn shared_resources_decode_identically_in_every_container() {
         bad.len()
     );
 }
+
+/// Port/starboard twins must not decode to the *same* buffer.
+///
+/// A runtime capture showed the container stores both halves of the `e106` hull
+/// as separate X-reflected buffers, so a `…_01`/`…_02` pair of equal vertex
+/// count should come out mirrored (or related by another axis / vertex order) —
+/// never identical, which is the collapse distinct assignment fixes. Disc-wide
+/// this holds for every such pair except `n206`, whose twins are grouped-pool
+/// resources (4 sub-meshes) and so are excluded from distinct assignment; that
+/// one is the remaining known case, asserted explicitly so it cannot grow.
+#[test]
+fn twin_pairs_do_not_share_a_buffer() {
+    let Some(root) = disc_root() else {
+        eprintln!("SKIP: extracted disc not found (set SYLPHEED_DISC to enable)");
+        return;
+    };
+    let mut files: Vec<PathBuf> = std::fs::read_dir(root.join("hidden/resource3d"))
+        .expect("resource3d/")
+        .flatten()
+        .map(|e| e.path())
+        .filter(|p| p.extension().and_then(|s| s.to_str()) == Some("xpr"))
+        .collect();
+    files.sort();
+
+    let mut collapsed: Vec<String> = Vec::new();
+    for f in &files {
+        let Ok(bytes) = std::fs::read(f) else { continue };
+        let models = Xbg7Model::anchor_models_cancellable(&bytes, 0.0, &|| false);
+        let by_name: BTreeMap<&str, &Xbg7Model> =
+            models.iter().map(|m| (m.name.as_str(), m)).collect();
+        for m in &models {
+            let Some(stem) = m.name.strip_suffix("_01") else { continue };
+            let Some(t) = by_name.get(format!("{stem}_02").as_str()) else { continue };
+            let (a, b) = (m.meshes[0].vbuf_offset, t.meshes[0].vbuf_offset);
+            if a.is_some() && a == b {
+                collapsed.push(format!("{}/{stem}_02 in {}", m.name, f.file_name().unwrap().to_string_lossy()));
+            }
+        }
+    }
+    collapsed.retain(|c| !c.starts_with("n206_01"));
+    assert!(collapsed.is_empty(), "twin pairs sharing one buffer: {collapsed:?}");
+}