feat(xbg7): read the per-sub-mesh vertex declarations (XBG7_SUBMESH_DECLS, off by default)

desc_dump shows each index marker is followed by its OWN element triples: n201_01
declares strides 24, 24, 24 and 28 (the last sub-mesh has a fourth element), which
matches the runtime capture's stride=28 on that draw and the four distinct vertex
shaders. parse_vertex_decl read the first declaration for the whole pool.

all_vertex_decls reads one per marker; anchor_grouped_meshes uses each sub-mesh's
own stride for the pool walk, the pivot validation and the read. debug_grouped_report
follows the same setting so the diagnostic cannot blame the wrong gate — at n201_01's
capture-proven pool start it now reports "pad 0: ACCEPTED" instead of a NaN position.

With XBG7_SUBMESH_DECLS=1: resources that never decode 85 -> 47, resources decoding
in no container 63 -> 30, capture oracles unchanged (93/93 index runs, 42/42 index
counts), consistency unchanged at 96.

Off by default because selection has not caught up: the three n201_0x copies then
settle on one pool (twin_pairs_do_not_share_a_buffer fails), production still picks
a start 4 bytes before sub-mesh #1 rather than the proven one even though the proven
start validates and is unclaimed, and four newly decoded ptc_pack .dat composites
carry degenerate triangles. Format truth is settled; choosing among candidates is
the remaining work.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NptfmpjdpNCKEez6d2xvA9

crates/sylpheed-formats/examples/starts_probe.rs

@@ -0,0 +1,7 @@
+fn main(){let a:Vec<String>=std::env::args().collect();let b=std::fs::read(&a[1]).unwrap();
+let stride:usize=a[2].parse().unwrap(); let want:usize=a[3].parse().unwrap();
+let s=sylpheed_formats::mesh::debug_vertex_run_starts(&b,stride);
+println!("{} candidate starts at stride {}", s.len(), stride);
+println!("contains {:#x}: {}", want, s.contains(&want));
+let near:Vec<String>=s.iter().filter(|&&o| o.abs_diff(want)<0x200).map(|o|format!("{o:#x}")).collect();
+println!("nearby: {}", near.join(" "));}