perf(streaming): cache per-column surface heights to fix move lag spikes

RecomputeTargetSet sampled TerrainSampler.HeightAt (a 169-point biome
blend kernel) 5x per column over the whole render distance on the main
thread every chunk crossing (~1,445 calls = ~250k biome lookups), causing
multi-frame hitches while moving. Memoize each chunk column's (min,max)
surface range, reused across ticks and pruned to the horizontal window, so
each column is sampled once. Exact (no terrain change); spike is amortized
to a thin edge strip per crossing and zero when stationary.

VoxelEngine.Tests/Streaming/ChunkStreamerTests.cs

@@ -447,6 +447,29 @@ public class ChunkStreamerTests
         Assert.Null(world.GetChunk(new Vector3i(0, 5, 0)));    // air above surface — not loaded
     }
 
+    [Fact]
+    public void RecomputeTargetSet_CachesSurfaceHeights_OnlySamplesNewColumns()
+    {
+        var cfg = new Configuration
+        {
+            RenderDistanceHorizontal = 1,
+            RenderDistanceVertical = 0,
+            SpawnReadyRadiusHorizontal = 1,
+            MaxGenerateJobsInFlight = 100,
+            MaxMeshUploadsPerFrame = 100,
+        };
+        int calls = 0;
+        var (streamer, _, _) = CreateStreamer(cfg, surfaceHeightAt: (_, _) => { calls++; return 8; });
+
+        streamer.Tick(new Vector3i(0, 0, 0));
+        int afterFirst = calls;                 // 9 columns × 5 samples = 45 (spawn-check reuses cache)
+        streamer.Tick(new Vector3i(1, 0, 0));   // overlaps 6 columns; only the cx=2 strip is new
+        int newCalls = calls - afterFirst;
+
+        Assert.Equal(45, afterFirst);
+        Assert.Equal(15, newCalls);             // 3 new columns × 5 — cached columns are not re-sampled
+    }
+
     [Fact]
     public void DrawVisible_DrawsOnlyChunksPassingPredicate_AndReturnsCount()
     {