revert(mesh): withdraw the neighbourhood anchor -- it regressed the e106 twin mirror

The neighbourhood anchor (f18d591) and its refinement (27a0701) took
cross-container inconsistency from 125 to 51 with coverage unchanged, and made
e106 render as a destroyer rather than a slab. Both are reverted.

ship::tests::static_assembly_matches_runtime_capture is gated on SYLPHEED_ISO, so
it SKIPS in an ordinary cargo test -- which is why the regression was invisible
in every suite run so far. With the ISO it fails:

  e106_bdy_01: static M row0 [-1.0, 0.0, 0.0] != captured [1.0, 0.0, 0.0]

e106_bdy_01 and _02 are a mirrored pair whose two buffers hold the same geometry
reflected in X, and BOTH resources currently decode to the SAME buffer (identical
counts, span and mean_x). apply_twin_mirrors picks which instance to reflect from
the sign of that mean_x, so which buffer wins flips the decision:

  before  both twins mean_x = -66.83  -> mirror bdy_02  (matches the capture)
  after   both twins mean_x = +66.83  -> mirror bdy_01  (contradicts it)

Neither is right -- two resources sharing one decode is itself the bug and the
mirror heuristic has been compensating. The capture is ground truth, so a change
that contradicts it does not ship. The real fix must give each twin its own
buffer first.

Kept from the attempt: this test now also asserts the SET of static placements
against the capture (allow-list {e303_wep_01} for vbase dedup), so extra
placements can finally fail it -- the direction it could never fail in before.

Docs, backlog, INDEX and the ignored test's message all corrected to say
diagnosed-not-fixed rather than fixed.

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

crates/sylpheed-formats/tests/mesh_consistency_disc.rs

@@ -53,7 +53,7 @@ fn span(m: &Xbg7Model) -> Option<[i64; 3]> {
 }
 
 #[test]
-#[ignore = "known-failing: 51 of 681 shared resources still decode inconsistently (125 before the neighbourhood anchor, 63 before refining it — 2026-08-12)"]
+#[ignore = "known-failing: 125 of 681 shared resources decode inconsistently. A neighbourhood anchor took this to 51 but regressed the e106 twin-mirror decision and was withdrawn — see docs/re/structures/xbg7-mesh.md"]
 fn shared_resources_decode_identically_in_every_container() {
     let Some(root) = disc_root() else {
         eprintln!("SKIP: extracted disc not found (set SYLPHEED_DISC to enable)");