re: n206 was fixed after all -- the audit was classifying by geometry, not buffer

debug_grouped_report (why_rejected) shows n206_02's alternative pool is ACCEPTED
at pad 0 under production gates and is in the candidate list -- and the decoder
does take it: n206_02 now anchors at 0x342d984. The 'still collapsed' reading
came from the audit comparing decoded geometry, and that offset holds a direct
(unmirrored) copy, so a separated pair still looked identical. The audit now
requires a SHARED BUFFER to call it a collapse: disc-wide 18 exact mirrors, 16
related, 0 collapses, 0 unrelated -- and the regression test drops its exception.

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

crates/sylpheed-formats/examples/starts.rs

@@ -0,0 +1,13 @@
+use sylpheed_formats::mesh::{debug_resource_params, debug_vertex_run_starts};
+fn main() {
+    let a: Vec<String> = std::env::args().collect();
+    let bytes = std::fs::read(&a[1]).unwrap();
+    let (markers, stride) = debug_resource_params(&bytes, &a[2]).expect("resource");
+    println!("{}: stride={stride} markers={markers:?}", a[2]);
+    let starts = debug_vertex_run_starts(&bytes, stride);
+    println!("{} candidate starts at stride {stride}", starts.len());
+    for off in &a[3..] {
+        let o = usize::from_str_radix(off.trim_start_matches("0x"), 16).unwrap();
+        println!("  0x{o:x} in candidate list: {}", starts.binary_search(&o).is_ok());
+    }
+}

crates/sylpheed-formats/examples/twin_mirror_audit.rs

@@ -74,6 +74,15 @@ fn main() {
                 related += 1;
                 continue;
             }
+            // Identical GEOMETRY is only a collapse if it also comes from the
+            // same buffer: a container may store a part twice unmirrored, and
+            // then two twins on two copies decode identically and correctly.
+            let shared_buffer = m.meshes[0].vbuf_offset.is_some()
+                && m.meshes[0].vbuf_offset == t.meshes[0].vbuf_offset;
+            if ident && !shared_buffer {
+                related += 1;
+                continue;
+            }
             if ident {
                 same += 1;
                 if examples.len() < 6 {
@@ -101,7 +110,7 @@ fn main() {
     }
     println!("twin pairs of equal vertex count: {}", same + mirrored + unrelated + related);
     println!("  exact X-mirror (expected) : {mirrored}");
-    println!("  IDENTICAL  (collapse)     : {same}");
+    println!("  IDENTICAL, one buffer (collapse) : {same}");
     println!("  related other way (Y/Z mirror, reordered): {related}");
     println!("  unrelated  (mis-anchor?)  : {unrelated}");
     for e in examples {

crates/sylpheed-formats/examples/why_rejected.rs

@@ -0,0 +1,14 @@
+//! Why does the decoder refuse a grouped-pool resource at a given pool start?
+//! Usage: why_rejected <container.xpr> <resource>@<vb0-hex>...
+use sylpheed_formats::mesh::debug_grouped_report;
+fn main() {
+    let a: Vec<String> = std::env::args().collect();
+    let bytes = std::fs::read(&a[1]).expect("container");
+    for pair in &a[2..] {
+        let (name, off) = pair.split_once('@').expect("name@hex");
+        let vb0 = usize::from_str_radix(off.trim_start_matches("0x"), 16).expect("hex");
+        for line in debug_grouped_report(&bytes, name, vb0) {
+            println!("{line}");
+        }
+    }
+}