perf(mesh): memoise the whole-container decode

Filtering after the assignment made every subset query a full decode (~15s on a
50MB container). full_decode_cached memoises it per container -- fingerprint is
length plus three sampled 4KB windows, keyed with min_consistency, last four kept.
Decoding five ships from Stage_S02 in turn: 10.5s for the first, then 48us-1.4ms.
A stage now costs one decode rather than one per ship. Ten suites green, viewer
builds, and a spot-checked resource still lands on the same offset.

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

crates/sylpheed-formats/examples/bench_ships.rs

@@ -0,0 +1,22 @@
+//! Time repeated per-ship decodes of one container, as the viewer does.
+use std::collections::HashSet;
+use std::time::Instant;
+use sylpheed_formats::mesh::Xbg7Model;
+use sylpheed_formats::ship::{is_base_part, ship_id_of};
+fn main() {
+    let a: Vec<String> = std::env::args().collect();
+    let bytes = std::fs::read(&a[1]).unwrap();
+    let names = sylpheed_formats::mesh::xbg7_resource_names(&bytes);
+    let ids: Vec<String> = {
+        let mut v: Vec<String> = names.iter().filter(|n| is_base_part(n))
+            .filter_map(|n| ship_id_of(n).map(|s| s.to_string())).collect();
+        v.sort(); v.dedup(); v.truncate(5); v
+    };
+    for id in &ids {
+        let want: HashSet<String> = names.iter()
+            .filter(|n| ship_id_of(n) == Some(id.as_str())).cloned().collect();
+        let t = Instant::now();
+        let got = Xbg7Model::models_named(&bytes, &want, &|| false);
+        println!("{id}: {} models in {:?}", got.len(), t.elapsed());
+    }
+}