re: coverage is 96.4% (6069/6294) -- and the 225 misses get a gate breakdown

undecoded.rs supplies the denominator the coverage numbers never had; the disc
holds 6294 XBG7 resources, 6069 decode, 225 are searched and missed, 0 lack a
descriptor. gate_histogram.rs attributes each miss to the furthest gate its best
candidate reached: 120 connectivity, 74 grouped-pool (different path), 15
degenerate/extent, 9 winding, 7 buffer-not-covered. Recorded as a work-list, not
a verdict -- a wrong candidate can pass more gates than the true block.

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

crates/sylpheed-formats/examples/gate_histogram.rs

@@ -0,0 +1,60 @@
+//! Which gate stops the resources that never decode?
+//! Usage: gate_histogram <resource3d_dir> [max_resources]
+use sylpheed_formats::mesh::{debug_best_rejection, xbg7_resource_names, Xbg7Model};
+use std::collections::{BTreeMap, HashSet};
+
+fn main() {
+    let dir = std::env::args().nth(1).expect("resource3d dir");
+    let cap: usize = std::env::args().nth(2).and_then(|s| s.parse().ok()).unwrap_or(usize::MAX);
+    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();
+
+    let mut hist: BTreeMap<usize, (usize, String)> = BTreeMap::new();
+    let mut done = 0usize;
+    for f in &files {
+        if done >= cap {
+            break;
+        }
+        let Ok(bytes) = std::fs::read(f) else { continue };
+        let names = xbg7_resource_names(&bytes);
+        if names.is_empty() {
+            continue;
+        }
+        let got: HashSet<String> = Xbg7Model::anchor_models_cancellable(&bytes, 0.0, &|| false)
+            .into_iter()
+            .map(|m| m.name)
+            .collect();
+        for n in names.iter().filter(|n| !got.contains(*n)) {
+            if done >= cap {
+                break;
+            }
+            done += 1;
+            if let Some((rank, why)) = debug_best_rejection(&bytes, n) {
+                let e = hist.entry(rank).or_insert((0, String::new()));
+                e.0 += 1;
+                if e.1.is_empty() {
+                    e.1 = format!("{n}: {why}");
+                }
+            }
+        }
+    }
+    println!("furthest gate reached, over {done} resources that never decode:");
+    let label = |r: usize| match r {
+        0 => "no gate reached",
+        1 => "index out of range",
+        2 => "buffer not covered by indices",
+        3 => "degenerate / implausible positions",
+        4 => "connectivity (mean edge / diagonal)",
+        5 => "winding consistency",
+        9 => "grouped pool (different path)",
+        _ => "?",
+    };
+    for (r, (n, ex)) in &hist {
+        println!("  {:<38} {n:>5}   e.g. {ex}", label(*r));
+    }
+}