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());
+    }
+}