Fix terrain streaming and surface spawning

internal/world/cache_light_reuse_test.go

@@ -0,0 +1,35 @@
+package world
+
+import (
+	"sync"
+	"testing"
+)
+
+func TestAdjacentFramesReuseGeneratedLightNeighbors(t *testing.T) {
+	var mu sync.Mutex
+	generated := make(map[[2]int32]int)
+	cache := NewCache(-1, func(cx, cz int32) *Chunk {
+		mu.Lock()
+		generated[[2]int32{cx, cz}]++
+		mu.Unlock()
+		return NewChunk(cx, cz, BiomePlains)
+	})
+
+	if _, err := cache.FrameErr(0, 0); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := cache.FrameErr(1, 0); err != nil {
+		t.Fatal(err)
+	}
+
+	mu.Lock()
+	defer mu.Unlock()
+	if len(generated) != 12 {
+		t.Fatalf("generated chunks = %d, want 12 shared neighborhood chunks", len(generated))
+	}
+	for pos, count := range generated {
+		if count != 1 {
+			t.Fatalf("chunk %v generated %d times, want once", pos, count)
+		}
+	}
+}