wip(assembler): round S13 -- tile cost counted per LOD (diagnostics only, no output change)

Exclusive per-stage tile timers and probes. Findings: LOD0 is MC-exclusive
(10.4 of 14 ms p95, 243 ns/sample); LOD4's 3x cost is column-cache misses
(8,224 vs ~1,600-1,900 at LOD3/5), cap refine secondary; per-tile feature lists
cost ~0.001 ms; cross-worker column repeats common (277k at LOD0). Global octave
drop and stride-2 interpolation probes fail the 2 cm gate. Worker cap 1 kept
(cap 0 worse 40 m/s tick tail). Report: Standalone/Research/WorldAssemblerM3/Round17.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MmSoF64BbRFmk9C3p1VsBJ

Standalone/Core/src/WorldAssemblerInternal.h

@@ -1027,11 +1027,18 @@ namespace world_assembly_detail
                                           std::int64_t wavelengthSubvoxels)
     { return FloorDiv(positionSubvoxels * Q16, wavelengthSubvoxels); }
 
+    // Audit-only override; normal tile generation resets this to zero before evaluating a recipe.
+    // Surcharge reservee aux mesures; la generation remet zero avant d'evaluer une recette.
+    inline thread_local std::uint32_t HeightfieldColumnOctaveDrop = 0;
+
     inline std::int32_t FractalNoiseQ16(std::int64_t xQ16, std::int64_t yQ16,
         std::int64_t zQ16, std::uint32_t octaves, bool ridged = false)
     {
         std::int64_t total = 0, weightSum = 0, weight = Q16, ridgeWeight = Q16;
-        for (std::uint32_t octave = 0; octave < octaves; ++octave)
+        std::uint32_t effectiveOctaves=octaves;
+        if(HeightfieldColumnOctaveDrop!=0&&octaves!=0)
+            effectiveOctaves=std::max(1u,octaves-std::min(octaves-1,HeightfieldColumnOctaveDrop));
+        for (std::uint32_t octave = 0; octave < effectiveOctaves; ++octave)
         {
             const std::int64_t scale = 1ll << octave;
             const std::int64_t noise = Perlin3DQ16(xQ16 * scale,yQ16 * scale,zQ16 * scale);
@@ -1965,9 +1972,15 @@ HeightfieldPointSample Heightfield2D::point(std::int64_t x, std::int64_t y,
     return out;
 }
 
-void Heightfield2D::resetThreadColumnCacheStats(bool captureCoordinates) noexcept
+void Heightfield2D::resetThreadColumnCacheStats(bool captureCoordinates, bool clearCache,
+    std::uint32_t octaveDrop) noexcept
 {
+    // Diagnostic probes may clear old tile entries before comparing isolated sample grids.
+    // Les sondes de mesure peuvent vider les entrees d'une tuile avant de comparer deux grilles.
+    if(clearCache)world_assembly_detail::HeightfieldColumnCache.fill(
+        world_assembly_detail::HeightfieldColumnCacheEntry{});
     auto& Stats=world_assembly_detail::HeightfieldColumnStats;
+    world_assembly_detail::HeightfieldColumnOctaveDrop=octaveDrop;
     Stats.pointCalls=0;
     Stats.columnHits=0;
     Stats.columnMisses=0;