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/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}");
+        }
+    }
+}