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/examples/index_hash_dump.rs

@@ -0,0 +1,32 @@
+//! Dump a fingerprint of every decoded index run, so two decoder settings can be
+//! diffed exactly. Usage: index_hash_dump <resource3d_dir>
+use sylpheed_formats::mesh::Xbg7Model;
+
+fn main() {
+    let dir = std::env::args().nth(1).expect("resource3d dir");
+    let mut files: Vec<_> = std::fs::read_dir(&dir)
+        .unwrap()
+        .flatten()
+        .map(|e| e.path())
+        .filter(|p| p.extension().and_then(|s| s.to_str()) == Some("xpr"))
+        .collect();
+    files.sort();
+    for f in &files {
+        let Ok(bytes) = std::fs::read(f) else { continue };
+        let w = f.file_name().unwrap().to_string_lossy().to_string();
+        for m in Xbg7Model::stage_models(&bytes) {
+            for (k, sm) in m.meshes.iter().enumerate() {
+                let mut h = 1469598103934665603u64;
+                for i in &sm.indices {
+                    h = (h ^ *i as u64).wrapping_mul(1099511628211);
+                }
+                println!(
+                    "{w} {} {k} vb={:?} n={} h={h:016x}",
+                    m.name,
+                    sm.vbuf_offset,
+                    sm.indices.len()
+                );
+            }
+        }
+    }
+}