re: calibrate the connectivity cap -- +612 resources against 18 consistency regressions

XBG7_EDGE_CAP / XBG7_SMALL_TRIS make the threshold sweepable (defaults unchanged,
full suite green). Above 0.417 the capture-proven eng_02_l anchors exactly right
and no e106 part regresses, and nothing that decoded at 0.28 is lost -- but ~250
existing anchors move silently and 18 shared resources lose cross-container
consistency. Not changed: the movers have no oracle yet.

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

crates/sylpheed-formats/src/mesh.rs

@@ -708,6 +708,27 @@ pub fn debug_find_index_buffer(bytes: &[u8], name: &str, vb: usize) -> Vec<(usiz
     out
 }
 
+/// The connectivity cap: a searched block whose mean triangle edge exceeds this
+/// fraction of its bounding-box diagonal is rejected. `0.28` is the shipped
+/// value. A runtime capture names blocks the engine really draws, and one of them
+/// (`e106_eng_02_l`, a 24-triangle LOD) measures **0.417** — so the cap has a
+/// known false positive. `XBG7_EDGE_CAP` overrides it for calibration sweeps;
+/// it is a diagnostic knob, not a setting (see docs/re/structures/xbg7-mesh.md).
+fn edge_cap() -> f32 {
+    std::env::var("XBG7_EDGE_CAP").ok().and_then(|v| v.parse().ok()).unwrap_or(0.28)
+}
+
+/// Triangle count below which the looser [`small_cap`] applies. `0` (default)
+/// disables the split, so the flat [`edge_cap`] governs every block.
+fn small_tris() -> usize {
+    std::env::var("XBG7_SMALL_TRIS").ok().and_then(|v| v.parse().ok()).unwrap_or(0)
+}
+
+/// The connectivity cap for blocks below [`small_tris`] triangles.
+fn small_cap() -> f32 {
+    std::env::var("XBG7_EDGE_CAP_SMALL").ok().and_then(|v| v.parse().ok()).unwrap_or(0.45)
+}
+
 /// Internal: the descriptor parameters the diagnostics need.
 fn decl_of(bytes: &[u8], name: &str) -> Option<(VertexDecl, Vec<(usize, usize)>)> {
     if bytes.len() < 16 || &bytes[..4] != b"XPR2" {
@@ -987,7 +1008,13 @@ fn validate_block(
             .sqrt()
             .max(1e-6);
         let mean_edge = edge_sum / (sampled as f32 * 3.0);
-        if mean_edge / diag > 0.28 {
+        // A COARSE block is coarse by construction: a 24-triangle LOD's edges
+        // are a large fraction of its own size, which is why the flat cap has a
+        // capture-proven false positive (`e106_eng_02_l`, ratio 0.417). Under
+        // `XBG7_SMALL_TRIS` blocks below that triangle count get the looser
+        // `XBG7_EDGE_CAP_SMALL` instead — a targeted relaxation, off by default.
+        let cap = if tris < small_tris() { small_cap() } else { edge_cap() };
+        if mean_edge / diag > cap {
             return false;
         }
     }