perf(core): tile-drain instrumentation; parallel FI sign-edge repair, FI first tile 1.28 -> 0.85 s (L55); R24

Opt-in --instrument-tile-drain records per-tile queue/dispatch/finish timelines, stage spans and counters
(proof skips, X8 lanes, GradAt/midpoint memo, skirts, FI cache stats); <2% when off. Measured: SW 13.3 and
FI 50.2 worker-s across LOD0-8, tails are work-saturated. FI sign-edge repair now computes in parallel and
publishes in sorted serial order (exact output): sign-edge 535 -> 47 ms, FI first tile 1.28 -> 0.85 s, near
ring 1.46 -> 1.04 s, whole view 6.09 -> 5.65 s. Proof block size, lane batching, SW midpoint memo and helper
cap 12 measured and left off. fast/medium/full PASS, Clang and GCC. Research R24: slab + motif generator.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016dhxM43dnWvUu2B5Ph1iG1

Standalone/Core/src/FloatingIslandsInternal.h

@@ -471,6 +471,26 @@ namespace floating_island_detail
         double DeltaX=0.0,DeltaY=0.0,Radial=0.0,Angle=0.0,LobePhase=0.0,TopSurface=0.0;
     };
 
+    struct FIslandColumnCacheInstrumentation
+    {
+        std::uint64_t localHits=0,localMisses=0,localConflictReplacements=0;
+        VoxelForgeCoreShared::FColumnCacheMetrics shared;
+    };
+
+    inline FIslandColumnCacheInstrumentation*& ActiveIslandColumnCacheInstrumentation() noexcept
+    {
+        static thread_local FIslandColumnCacheInstrumentation* active=nullptr;
+        return active;
+    }
+
+    inline FIslandColumnCacheInstrumentation* SetIslandColumnCacheInstrumentation(
+        FIslandColumnCacheInstrumentation* value) noexcept
+    {
+        FIslandColumnCacheInstrumentation* previous=ActiveIslandColumnCacheInstrumentation();
+        ActiveIslandColumnCacheInstrumentation()=value;
+        return previous;
+    }
+
     using FSharedIslandColumnCache =
         VoxelForgeCoreShared::TSharedColumnValueCache<FIslandColumnCacheEntry,16384,4>;
 
@@ -501,6 +521,7 @@ namespace floating_island_detail
         IndexKey^=IndexKey>>33;
         const std::size_t Base=static_cast<std::size_t>(IndexKey&(SetCount-1))*Ways;
         FIslandColumnCacheEntry* Replacement=&cache[Base+((IndexKey>>11)&(Ways-1))];
+        bool bFoundInvalid=false;
         for(std::size_t Way=0;Way<Ways;++Way)
         {
             FIslandColumnCacheEntry& Entry=cache[Base+Way];
@@ -516,8 +537,18 @@ namespace floating_island_detail
                 &&Entry.TopHalf==island.topHalfSubvoxels
                 &&Entry.TopFlattenQ16==recipe.topFlattenQ16
                 &&Entry.ContourWarp==recipe.contourWarpSubvoxels;
-            if(bKeyMatches)return Entry;
-            if(!Entry.bValid)Replacement=&Entry;
+            if(bKeyMatches)
+            {
+                if(auto* stats=ActiveIslandColumnCacheInstrumentation())++stats->localHits;
+                return Entry;
+            }
+            if(!Entry.bValid&&!bFoundInvalid)
+            {Replacement=&Entry;bFoundInvalid=true;}
+        }
+        if(auto* stats=ActiveIslandColumnCacheInstrumentation())
+        {
+            ++stats->localMisses;
+            if(!bFoundInvalid)++stats->localConflictReplacements;
         }
 
         const auto build=[&]()
@@ -562,7 +593,9 @@ namespace floating_island_detail
                 static_cast<std::uint64_t>(reinterpret_cast<std::uintptr_t>(&plan)),
                 plan.contentHash,SampleX,SampleY,island.stableId};
             bool computed=false;
-            Entry=shared->getOrCompute(key,build,computed);
+            auto* stats=ActiveIslandColumnCacheInstrumentation();
+            Entry=shared->getOrCompute(key,build,computed,
+                stats?&stats->shared:nullptr);
         }
         else Entry=build();
         *Replacement=Entry;