bench(sqlite): add isolated cache rebuild benchmarks (bd-zw72)

Add BenchmarkRebuildBlockedCache_Large and _XLarge to measure cache
rebuild performance in isolation.

Results show cache rebuild takes ~773ms at 10K issues and ~1.3s at 20K,
significantly slower than the ~50ms originally estimated in the issue.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/sqlite/sqlite_bench_test.go

@@ -236,6 +236,39 @@ func BenchmarkSyncMerge(b *testing.B) {
 	}
 }
 
+// BenchmarkRebuildBlockedCache_Large benchmarks cache rebuild in isolation on 10K database
+// This measures the core operation that bd-zw72 is investigating for incremental optimization
+func BenchmarkRebuildBlockedCache_Large(b *testing.B) {
+	store, cleanup := setupLargeBenchDB(b)
+	defer cleanup()
+	ctx := context.Background()
+
+	b.ResetTimer()
+	b.ReportAllocs()
+
+	for i := 0; i < b.N; i++ {
+		if err := store.rebuildBlockedCache(ctx, nil); err != nil {
+			b.Fatalf("rebuildBlockedCache failed: %v", err)
+		}
+	}
+}
+
+// BenchmarkRebuildBlockedCache_XLarge benchmarks cache rebuild in isolation on 20K database
+func BenchmarkRebuildBlockedCache_XLarge(b *testing.B) {
+	store, cleanup := setupXLargeBenchDB(b)
+	defer cleanup()
+	ctx := context.Background()
+
+	b.ResetTimer()
+	b.ReportAllocs()
+
+	for i := 0; i < b.N; i++ {
+		if err := store.rebuildBlockedCache(ctx, nil); err != nil {
+			b.Fatalf("rebuildBlockedCache failed: %v", err)
+		}
+	}
+}
+
 // Helper function
 func intPtr(i int) *int {
 	return &i