perf(gen): interval propagation -- blocks can finally be PROVEN empty. 1.55s -> 1.21s

Task 4 ran 11 hours and was killed by system memory pressure before it could file a
report. The work survived, builds clean (0 errors), and is verified faster with
byte-identical output. This commit is my verification, not its summary -- it never
got to write one.

THE STRUCTURAL FIX
0e9df9e established why no block could ever be skipped: OriginSpineOp,
PassageCarveOp and the active seals Lerp/Min/Max against the CURRENT density, so
with an unknown incoming density their supremum is genuinely unbounded. Not a
missing bound -- the operator semantics.

The answer is to propagate a density INTERVAL through the stack, and that is what
landed: FVoxelBoxSdfInterval + PropagateSdfOverBox on the operator interface, an
interval fold through every converter and modifier, interval arithmetic down into
the noise primitives themselves (gradient corners, warp, worm frequencies), and a
BlockSkip lattice the mesher consults per 8^3 block.

Its own guiding comment, which is the right default:
  "no finite interval is a proof. Unknown costs the skip and protects the geometry."

  128^3 TunnelNetwork seed 1337 slot 1, freshly built binary, 3 runs each:
    before  1.515 / 1.554 / 1.569 s
    after   1.213 / 1.258 / 1.171 s     -22.5%, 1.28x -- clears the 17% floor

  Geometry SHA-256 98eaad61..d3c4f IDENTICAL to 0e9df9e. 153,740 triangles either
  way. It skips only provably-empty work; nothing about the world changed.

It also fixed one cause of the 5.6 s empty tiles found in 0e9df9e: an LOD4 tile
legitimately spans 19^3 = 6,859 chunk keys and was rebuilding per-chunk state
thousands of times within a single tile. A shared cache removed that failure mode;
remaining slow empties were LOD0 and are a separate fast-classifier bail, still open.

Partially landed, unfinished when killed: RequestStartCycles on FChunkResult, to
report request-to-ready latency rather than worker generation time. Harmless as is.

⚠️ WHAT KILLED IT, AND IT IS OURS
A single UnrealEditor-Cmd commandlet grew to 19.5 GB, leaving 1.3 GB free on a
32 GB machine, and the OS killed the agent. Freeing it restored 21.6 GB. The owner
said twice that his machine should not be under memory pressure and he was right
both times: first the 411 MiB worker caches, now a 19.5 GB commandlet. A generation
run must not need 19.5 GB. Next task.

Cumulative on this region: 342.2 -> 16.2 -> 2.62 -> 1.49 -> 1.21 s  (~283x).
3,374 insertions / 342 deletions across 12 files. Build 0 errors. Nothing outside
Plugins/VoxelForge; RealtimeMeshComponent untouched.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012aheo2crrZmHuYtPpdY2Cb

Source/VoxelForge/Private/VoxelCaveMorphology.cpp

@@ -33,6 +33,44 @@
 #include "VoxelStrateMeasure.h"
 #include "VoxelTypes.h"          // Pour VOXEL_NOISE_SCALE, SmoothStep01
 #include "VoxelStrateTypes.h"
+#include "HAL/PlatformTime.h"
+
+namespace
+{
+    struct FTask4BuildProbeScope
+    {
+        FChunkSDFCache& Cache;
+        const float MinX;
+        const float MinY;
+        const float MaxX;
+        const float MaxY;
+        const uint64 StartCycles;
+        const bool bEnabled;
+
+        FTask4BuildProbeScope(FChunkSDFCache& InCache, float InMinX, float InMinY,
+                               float InMaxX, float InMaxY)
+            : Cache(InCache), MinX(InMinX), MinY(InMinY), MaxX(InMaxX), MaxY(InMaxY)
+            , StartCycles(FPlatformTime::Cycles64())
+            , bEnabled((InMaxX - InMinX) > 120.0f && (InMaxX - InMinX) < 220.0f
+                       && (InMaxY - InMinY) > 120.0f && (InMaxY - InMinY) < 220.0f
+                       && InMinX > -512.0f && InMaxX < 512.0f
+                       && InMinY > -512.0f && InMaxY < 512.0f)
+        {}
+
+        ~FTask4BuildProbeScope()
+        {
+            if (bEnabled)
+            {
+                UE_LOG(LogTemp, Warning,
+                       TEXT("[Task4BuildProbe] search=(%.0f,%.0f)-(%.0f,%.0f) rooms=%d joins=%d tunnels=%d pits=%d chimneys=%d ms=%.3f"),
+                       MinX, MinY, MaxX, MaxY, Cache.Rooms.Num(), Cache.RoomFloorJoins.Num(),
+                       Cache.Tunnels.Num(), Cache.Pits.Num(), Cache.Chimneys.Num(),
+                       FPlatformTime::ToMilliseconds64(
+                           FPlatformTime::Cycles64() - StartCycles));
+            }
+        }
+    };
+}
 #include "VoxelNoise.h"           // Pure FBM used by the slab landing query
 #include "VoxelPassageGeometry.h"
 #include "VoxelTerrainOpDefinition.h"
@@ -2828,6 +2866,8 @@ void VoxelCaveMorphology::BuildChunkCache(
     uint32 Seed, int32 StrateIndex,
     const TArray<FStrateTerrainOpEntry>* TerrainOps)
 {
+    FTask4BuildProbeScope Task4BuildProbe(OutCache, SearchMinX, SearchMinY, SearchMaxX, SearchMaxY);
+
     // Clear previous data (arrays keep their allocation for reuse)
     OutCache.Rooms.Reset();
     OutCache.RoomFloorJoins.Reset();