fix(mesh): require exact pool coverage -- found by rendering, not by a metric

Rendering assembled e106 from Stage_S02 showed the old slab: e106_bdy_03 spanning
600x1600x998 where three other containers give 276x236x941. Its anchor had slack
3, and the coverage gate tolerated up to three unreferenced tail vertices --
tolerance that was hiding a mis-anchor, since real blocks reach their last vertex
exactly (8580 of 8629). Requiring exact coverage moves it to the block the other
containers agree on and the slab disappears.

Costs 3 resources (6212 -> 6209), inconsistency 39 -> 38, capture oracle
unchanged at 46/46, suite green. Evidence: captures/e106-cover-slack-before-after.png

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

crates/sylpheed-formats/examples/submesh_dump.rs

@@ -11,13 +11,23 @@ fn main() {
         println!("{} — {} sub-meshes decoded, {} markers declared", m.name, m.meshes.len(), markers.len());
         for (i, s) in m.meshes.iter().enumerate() {
             let max_idx = s.indices.iter().max().copied().unwrap_or(0) as usize;
+            let (mut lo, mut hi) = ([f32::MAX; 3], [f32::MIN; 3]);
+            for q in &s.positions {
+                for k in 0..3 {
+                    lo[k] = lo[k].min(q[k]);
+                    hi[k] = hi[k].max(q[k]);
+                }
+            }
             println!(
-                "  #{i:<2} at 0x{:<9x} verts {:<6} idx {:<6} max_idx {:<6} slack {}",
+                "  #{i:<2} at 0x{:<9x} verts {:<6} idx {:<6} max_idx {:<6} slack {} span [{:.0} {:.0} {:.0}]",
                 s.vbuf_offset.unwrap_or(0),
                 s.positions.len(),
                 s.indices.len(),
                 max_idx,
-                s.positions.len() as i64 - 1 - max_idx as i64
+                s.positions.len() as i64 - 1 - max_idx as i64,
+                hi[0] - lo[0],
+                hi[1] - lo[1],
+                hi[2] - lo[2]
             );
         }
     }

crates/sylpheed-formats/src/mesh.rs

@@ -976,6 +976,21 @@ fn grouped_consistency() -> f32 {
     std::env::var("XBG7_GROUPED_CONSISTENCY").ok().and_then(|v| v.parse().ok()).unwrap_or(0.85)
 }
 
+/// Coverage requirement, as `max_index + N >= vtx_count`. **`1` since
+/// 2026-08-12** — i.e. the indices must reach the pool's last vertex exactly.
+///
+/// The old `4` tolerated three unreferenced tail vertices, and that slack was a
+/// mis-anchor tell rather than a real variation: 8 580 of 8 629 decoded
+/// sub-meshes cover their pool exactly, and `e106_bdy_03` in `Stage_S02` was one
+/// of the few that did not — slack 3, decoding to a 600×1600×998 slab visible in
+/// a render, where three other containers give 276×236×941. Requiring exact
+/// coverage moves it onto the block those containers agree on. Costs 3 resources
+/// disc-wide; capture oracle unchanged at 46/46. `XBG7_COVER_SLACK` overrides
+/// (note `0` rejects everything — the comparison is `max_idx + N >= vtx_count`).
+fn cover_slack() -> usize {
+    std::env::var("XBG7_COVER_SLACK").ok().and_then(|v| v.parse().ok()).unwrap_or(1)
+}
+
 /// Smallest bounding-box extent a block may have (default `0.5`). An absolute
 /// floor on a format with no unit convention is a scale assumption, so it is a
 /// knob: `XBG7_MIN_EXTENT`.
@@ -1262,7 +1277,7 @@ fn validate_block_report(
         }
         max_idx = max_idx.max(i);
     }
-    if (max_idx as usize) + 4 < vtx_count {
+    if (max_idx as usize) + cover_slack() < vtx_count {
         return Err(format!(
             "indices reach only {max_idx} of {vtx_count} vertices (buffer not covered)"
         ));
@@ -1525,7 +1540,7 @@ fn anchor_grouped_meshes(
                     max_idx = max_idx.max(i);
                     i < vc
                 });
-                if in_range && max_idx + 4 >= vc {
+                if in_range && max_idx + cover_slack() >= vc {
                     meshes.push(read_pool_mesh(bytes, ib, vb, ic, vc, decl));
                 }
                 vb += vc * stride;