perf(streaming): outer tile classifier off by default; delete mode 2

Owner-approved, on the A/B measured in 83a0e4d: the classifier plus exact validation cost more
than it saved on the game path. With it off, ready p95 is 182 vs 190-195 ms, generation p95 is
125-127 vs 139-141 ms, and density calls drop 18%.

- GVoxelForgeOuterClassifierMode defaults to 0. Mode 1 stays reachable via
  voxel.OuterClassifierMode=1 as an A/B instrument; the clamp is now 0..1.
- Mode 2 (validation samples reused as the mesher core grid) is deleted: the reuse grid,
  FVoxelMesherCoreDensityGrid, GenerateMesh's ReusableCoreGrid parameter, bValidationCoreReused,
  TotalValidationCoreReuseTiles and its log field. It could not fire: it reused the grid only
  when validation disproved a uniform verdict, which never happened, and Mixed tiles never
  validate. The mode-1 validation loop exits early again on the first disagreeing sample.
- All three GenerateMesh call sites checked; the density-count pointer binds after BandVoxHi.

Verified by Codex, checked by me against the logs:
- build ok; the measured process loaded the staged DLL, SHA-256 1024D8CF..04D
- export OBJ SHA-256 unchanged: b3e5f4c3..9b377
- a run WITHOUT the override prints outer_classifier_mode=0 validation_density_calls=0,
  proving the new default took effect
- 841/841 tiles have identical triangle counts between mode 0 and mode 1. The 394 tiles mode 1
  skipped as uniform produce 0 triangles in mode 0, so the strate band cut (applied by the
  mesher, not by validation) was not hiding geometry.

Docs: ARCHITECTURE 8.10, CODEMAP (GenerateTileResult, ClassifyTile) and fable-idea T1.d record
that streaming no longer classifies by default. WORK-NEXT item 1 records the owner's playtest:
he can no longer fall through the world.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VZS94UH8bSsBeg5sZaxe2Y

ARCHITECTURE.md

@@ -794,6 +794,13 @@ driven by `EditorBrush*` props.
   shadow-casting tile emits a second shadow-pass draw; the far coarse tiles don't need it. NOTE: fps is
   RENDER-side (draws ≈ visible tile count × passes); generation cost (workers) and tile *resolution*
   (cuts triangles, not draws/components) don't move the game thread — tile COUNT does (hence the clipmap).
+- ⛔ **2026-09-13: streaming no longer calls `ClassifyTile` by default** (`voxel.OuterClassifierMode`
+  = 0; `=1` restores it as an A/B instrument). Measured on the headless game path with nested
+  refinement off, the classifier plus its exact validation cost more than they saved. Mode 0 had
+  ready p95 182 vs 190-195 ms, generation p95 125-127 vs 139-141 ms, and 18% fewer density calls,
+  with 841/841 tiles triangle-identical (WORK-NEXT.md). Proving a 33³ core uniform costs almost
+  as much as meshing 35³. The history below explains why it existed. Its soundness tests still
+  guard it.
 - **Trivial-empty tile reject (T1.d) — v2 SHIPPED (2026-07-05); v1 was reverted 2026-06-26.**
   `UVoxelGenerator::ClassifyTile(Origin, Step, Cells)` runs on the gen worker BEFORE the density
   pre-sample (`LoadTile` task, scope `VoxelForge_ClassifyTile`); `AllSolid`/`AllAir` ⇒ GenerateMesh is