Increase chunkSize from 100 to 1000 to reduce lock contention

With chunkSize=100 and 10M items, 100K chunks cause ~300K mutex
lock/unlock operations per search across 32 goroutines competing
for a single sync.Mutex in ChunkCache.

Increasing to 1000 reduces chunks to 10K, cutting contention overhead.
Benchmarks on 10M items show 14-80% faster searches depending on query
selectivity.

src/constants.go

@@ -39,7 +39,7 @@ const (
 	progressMinDuration     = 200 * time.Millisecond
 
 	// Capacity of each chunk
-	chunkSize int = 100
+	chunkSize int = 1000
 
 	// Pre-allocated memory slices to minimize GC
 	slab16Size int = 100 * 1024 // 200KB * 32 = 12.8MB