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

Source/VoxelForge/Public/VoxelWorld.h

@@ -111,7 +111,6 @@ struct FChunkResult
     // count exact validation and mesher density calls locally, then publish one aggregate update.
     int64 ValidationDensityCalls = 0;
     int64 MesherDensityCalls = 0;
-    bool bValidationCoreReused = false;
     bool bAborted = false;    // Worker observed shutdown or cancellation; never mark this tile loaded
     bool bObsolete = false;   // Worker observed per-tile desired-set cancellation
     uint32 DesiredEpoch = 0;  // Desired-set epoch captured at request time
@@ -929,7 +928,6 @@ public:
     std::atomic<uint64> TotalObsoleteWorkerTasks{0};
     std::atomic<uint64> TotalValidationDensityCalls{0};
     std::atomic<uint64> TotalMesherDensityCalls{0};
-    std::atomic<uint64> TotalValidationCoreReuseTiles{0};
     std::atomic<uint64> OuterClassifierCallsByLOD[TrackedClassifierLODCount]{};
     std::atomic<uint64> OuterClassifierVerdictsByLOD[TrackedClassifierLODCount][3]{};