fix(xbg7): the index run was one element late for 575 sub-meshes

Extending the capture comparison from index COUNTS to index VALUES
(`examples/capture_index_bytes.rs`, using the batch offsets the new ib logging
gives) showed 76 of 93 Stage_S02 index runs identical to the GPU's and 17
differing — every difference a shift by exactly one element, on buffers whose
index data sits at pad 2.

`anchor_pool_mesh` returned the FIRST pad that validated, and pad 0 is tried
first with the looser winding gate (0.70 vs 0.85). Read at pad 0, a pad-2 block
yields [true[1], true[2], …, garbage]: every index in range, the pool covered,
the positions right, the winding often just above 0.70 — so it validated, and
every triangle was mis-wired. Nothing count-based could see it.

The signature is decidable without the capture: a shifted run wires arbitrary
vertices, so triangles come out degenerate. 282 of 283 correctly anchored
Stage_S02 blocks have zero degenerate triangles, while the shifted readings carry
1–2 156. So score every validating pad by (degenerate triangles, then winding)
and keep the best. `XBG7_PAD_FIRST_MATCH=1` restores the old behaviour.

  captured index runs identical:            76/93  ->  93/93  (2 025 elements)
  decoded runs with a degenerate triangle:    579  ->     16  (disc-wide)
  sub-meshes whose index run changed:                    575  of 8 850
  resources decoded / vertex anchors / consistency:  unchanged (6 209 / same vb / 89)

Locked in by tests/mesh_disc.rs::decoded_index_runs_have_almost_no_degenerate_triangles.
Suite green with --include-ignored apart from the pre-existing known-failing
cross-container consistency test (the 24-vertex bounding-box class).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NptfmpjdpNCKEez6d2xvA9

crates/sylpheed-formats/tests/mesh_disc.rs

@@ -424,3 +424,57 @@ fn stage_models_quality_audit() {
     assert!(huge < models.len() / 20, "few huge (skybox-plane) models");
     assert!(worst_deg < 0.35, "no model should be mostly-degenerate");
 }
+
+/// A correctly located index run has **no degenerate triangles**. That is the
+/// signature the 2026-08-13 pad-scoring fix keys on: an index list read one
+/// element late still passes every count-based gate but wires arbitrary
+/// vertices, which produces triangles with a repeated index. Before the fix
+/// **579** decoded runs on the disc carried such triangles (and a runtime
+/// capture confirmed 17 of 93 index batches disagreed with the GPU); after it,
+/// 16 sub-meshes do — all in resources that are separately known-broken. Locking the
+/// number in, because the defect is invisible to coverage and to the anchor
+/// oracle: every count stays correct while the geometry is mis-wired.
+#[test]
+#[ignore = "requires extracted disc models — set SYLPHEED_RES3D"]
+fn decoded_index_runs_have_almost_no_degenerate_triangles() {
+    let Some(dir) = res3d_dir() else {
+        eprintln!("SKIP: resource3d dir not found (set SYLPHEED_RES3D)");
+        return;
+    };
+    let mut files: Vec<PathBuf> = std::fs::read_dir(&dir)
+        .expect("resource3d/")
+        .flatten()
+        .map(|e| e.path())
+        .filter(|p| p.extension().and_then(|s| s.to_str()) == Some("xpr"))
+        .collect();
+    files.sort();
+
+    let mut offenders: Vec<(String, String, usize)> = Vec::new();
+    for f in &files {
+        let Ok(bytes) = std::fs::read(f) else { continue };
+        let where_ = f.file_name().unwrap().to_string_lossy().to_string();
+        for m in Xbg7Model::stage_models(&bytes) {
+            for sm in &m.meshes {
+                let d = sm
+                    .indices
+                    .chunks_exact(3)
+                    .filter(|t| t[0] == t[1] || t[1] == t[2] || t[0] == t[2])
+                    .count();
+                if d > 0 {
+                    offenders.push((m.name.clone(), where_.clone(), d));
+                }
+            }
+        }
+    }
+    // The known 16 sub-meshes: the grouped `.dat` break composites in `ptc_pack`
+    // (`f102`/`f104`/`e107`, whose marker lists are documented not to map onto the
+    // stored blocks — 5 sub-meshes), plus `e201_bdy_03_m` (2 containers) and
+    // `_rou_f402_dead` (9). Every one has an independently recorded anchoring
+    // problem; a NEW name appearing here is a regression.
+    assert!(
+        offenders.len() <= 16,
+        "{} decoded index runs contain degenerate triangles (expected ≤ 16): {:?}",
+        offenders.len(),
+        &offenders[..offenders.len().min(20)]
+    );
+}