fix(xbg7): degenerate index runs 582 -> 1 (grouped path + prefer a clean candidate)

Two follow-ups to the pad-scoring fix, both driven by the same invariant (a
correctly located index run has no degenerate triangles):

- anchor_grouped_meshes picked its pad by first-match too; scoring the pivot run
  the same way cleared every remaining ptc_pack composite (f102/f104/e107).
- anchor_pool_mesh now prefers a degenerate-free candidate over an earlier dirty
  one. examples/better_home.rs showed the last two resources each had exactly one
  degenerate-free, pool-covering block, sitting later in file order than the
  lookalike we took. First-match order is kept for every clean hit, and a dirty
  block is still used if nothing clean exists, so coverage cannot regress.

  degenerate index runs, disc-wide:      582 -> 11 -> 1
  captured index runs identical:         93/93 (unchanged)
  resources decoded / misses:            6 209 / 85 (unchanged)
  index runs changed / anchors moved:    590 / 10 (_rou_f402_dead x8, e201_bdy_03_m x2)

Cross-container minority decodes 89 -> 96, and that is progress: all seven new
rows are _rou_f402_dead, which now has a majority (32x25x8) for the first time, so
its seven wrong copies are named instead of hidden behind "no majority".

The last dirty run (_rou_f402_dead in Stage_S09) is blocked by distinct assignment
— its clean block is claimed by e_rou_f003_Near, both 24-vertex bounding boxes. A
winding-floor escalation for that case was written, measured to fire for nothing,
and reverted; the reasoning is kept as a comment.

Regression threshold tightened to 1. Suite green with --include-ignored apart from
the pre-existing known-failing cross-container consistency test.

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

crates/sylpheed-formats/examples/better_home.rs

@@ -0,0 +1,123 @@
+//! Does a resource have a CLEANER home in its container than the one we picked?
+//!
+//! After the pad-scoring fix, only two resources on the disc still decode to an
+//! index run with degenerate triangles — `e201_bdy_03_m` (2 containers) and
+//! `_rou_f402_dead` (9). Degeneracy says the run does not fit the pool, so either
+//! the vertex block is wrong or the candidate list never offered the right one.
+//! This walks every candidate vertex-run start for the resource's declaration and
+//! scores each `(start, pad)` the way the anchor now does — degenerate triangles
+//! first, then winding, plus coverage — so the answer is one of:
+//!   * a strictly cleaner candidate exists (the selection is at fault),
+//!   * several are equally clean (genuinely ambiguous), or
+//!   * nothing is clean (the block is not in the candidate list at all).
+//!
+//! Usage: better_home <container.xpr> <resource-name>
+use sylpheed_formats::mesh::{debug_resource_params, debug_vertex_run_starts, Xbg7Model};
+
+fn be16(b: &[u8], at: usize) -> u32 {
+    ((b[at] as u32) << 8) | b[at + 1] as u32
+}
+
+fn main() {
+    let a: Vec<String> = std::env::args().collect();
+    let bytes = std::fs::read(&a[1]).expect("container");
+    let name = &a[2];
+
+    let Some((markers, stride)) = debug_resource_params(&bytes, name) else {
+        eprintln!("no such XBG7 resource: {name}");
+        std::process::exit(1);
+    };
+    let (vc, ic) = markers[0];
+    println!("{name}: {} marker(s), first = {vc} verts / {ic} indices, stride {stride}", markers.len());
+
+    // Where did the decoder put it?
+    let ours = Xbg7Model::stage_models(&bytes)
+        .into_iter()
+        .find(|m| m.name == *name)
+        .and_then(|m| m.meshes.first().and_then(|s| s.vbuf_offset));
+    println!("our anchor: {ours:?}");
+
+    let starts = debug_vertex_run_starts(&bytes, stride);
+    println!("{} candidate vertex-run starts for stride {stride}", starts.len());
+
+    // Score every (start, pad): degenerate triangles, winding against the stored
+    // normals, and whether the run covers the pool exactly.
+    let mut rows: Vec<(usize, f32, usize, usize, usize, bool)> = Vec::new(); // degen, wind, start, pad, max_idx, covered
+    for &vb in &starts {
+        for pad in 0..=3usize {
+            if vb < ic * 2 + pad {
+                continue;
+            }
+            let ib = vb - ic * 2 - pad;
+            if ib + ic * 2 > bytes.len() || vb + vc * stride > bytes.len() {
+                continue;
+            }
+            let idx: Vec<u32> = (0..ic).map(|k| be16(&bytes, ib + k * 2)).collect();
+            let max_idx = *idx.iter().max().unwrap_or(&0) as usize;
+            if max_idx >= vc {
+                continue; // out of range — not a candidate at all
+            }
+            let pos: Vec<[f32; 3]> = (0..vc)
+                .map(|v| {
+                    let at = vb + v * stride;
+                    [
+                        f32::from_be_bytes(bytes[at..at + 4].try_into().unwrap()),
+                        f32::from_be_bytes(bytes[at + 4..at + 8].try_into().unwrap()),
+                        f32::from_be_bytes(bytes[at + 8..at + 12].try_into().unwrap()),
+                    ]
+                })
+                .collect();
+            if pos.iter().any(|p| p.iter().any(|c| !c.is_finite() || c.abs() > 1e6)) {
+                continue;
+            }
+            let mut degen = 0usize;
+            let (mut agree, mut counted) = (0usize, 0usize);
+            for t in idx.chunks_exact(3) {
+                let (x, y, z) = (t[0] as usize, t[1] as usize, t[2] as usize);
+                if x == y || y == z || x == z {
+                    degen += 1;
+                    continue;
+                }
+                // Winding needs normals; use the geometric centroid normal as a
+                // stand-in so this stays declaration-agnostic: a consistent mesh
+                // has all faces pointing away from the centroid on a convex-ish
+                // hull. Weak, so degeneracy leads the sort.
+                let e1 = [pos[y][0] - pos[x][0], pos[y][1] - pos[x][1], pos[y][2] - pos[x][2]];
+                let e2 = [pos[z][0] - pos[x][0], pos[z][1] - pos[x][1], pos[z][2] - pos[x][2]];
+                let f = [
+                    e1[1] * e2[2] - e1[2] * e2[1],
+                    e1[2] * e2[0] - e1[0] * e2[2],
+                    e1[0] * e2[1] - e1[1] * e2[0],
+                ];
+                let cx: [f32; 3] = {
+                    let mut c = [0.0f32; 3];
+                    for p in &pos {
+                        for k in 0..3 {
+                            c[k] += p[k] / pos.len() as f32;
+                        }
+                    }
+                    c
+                };
+                let out = [pos[x][0] - cx[0], pos[x][1] - cx[1], pos[x][2] - cx[2]];
+                counted += 1;
+                if f[0] * out[0] + f[1] * out[1] + f[2] * out[2] > 0.0 {
+                    agree += 1;
+                }
+            }
+            let w = if counted == 0 { 0.0 } else { agree as f32 / counted as f32 };
+            rows.push((degen, w.max(1.0 - w), vb, pad, max_idx, max_idx + 1 == vc));
+        }
+    }
+    rows.sort_by(|a, b| a.0.cmp(&b.0).then(b.1.total_cmp(&a.1)));
+    println!("\n{} in-range candidates; best 12 by (degenerate, winding):", rows.len());
+    for (d, w, vb, pad, mx, cov) in rows.iter().take(12) {
+        let mark = if Some(*vb) == ours { "  <-- ours" } else { "" };
+        println!(
+            "  vb 0x{vb:07X} pad {pad}  degen {d:>4}  wind {w:.3}  max_idx {mx}/{}  {}{mark}",
+            vc - 1,
+            if *cov { "covers" } else { "SHORT" }
+        );
+    }
+    let clean = rows.iter().filter(|r| r.0 == 0 && r.5).count();
+    println!("\n{clean} candidates are degenerate-free AND cover the pool exactly");
+}