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/pad_shift_audit.rs

@@ -67,6 +67,16 @@ fn main() {
         let where_ = f.file_name().unwrap().to_string_lossy().to_string();
         let mut c_moved = 0usize;
         for m in Xbg7Model::stage_models(&bytes) {
+            // Degeneracy is counted over EVERY sub-mesh (grouped pools included —
+            // they have their own pad choice); the pad-0 comparison below only
+            // applies to single-block resources, whose run starts at `vb - 2n`.
+            for sm in &m.meshes {
+                let d = degenerate(&sm.indices);
+                if d > 0 {
+                    still_degen += 1;
+                    worst.push((d, m.name.clone(), where_.clone()));
+                }
+            }
             if m.meshes.len() != 1 {
                 continue;
             }
@@ -98,11 +108,6 @@ fn main() {
                     weak_wind.push((w0, w2, m.name.clone()));
                 }
             }
-            let d = degenerate(&sm.indices);
-            if d > 0 {
-                still_degen += 1;
-                worst.push((d, m.name.clone(), where_.clone()));
-            }
         }
         if c_moved > 0 {
             println!("{where_:<24} {c_moved} resources read from a non-pad-0 index run");