Replace []Result cache with bitmap cache for reduced memory usage

Replace the per-chunk query cache from []Result slices to fixed-size
bitmaps (ChunkBitmap: [16]uint64 = 128 bytes per entry). Each bit
indicates whether the corresponding item in the chunk matched.

This reduces cache memory by 86 times in testing:
- Old []Result cache: ~22KB per chunk per query (for 500 matches)
- New bitmap cache:   ~262 bytes per chunk per query (fixed)

With the reduced per-entry cost, queryCacheMax is raised from
chunkSize/5 to chunkSize/2, allowing broader queries (up to 50% match
rate) to be cached while still using far less memory.

src/constants.go

@@ -37,14 +37,15 @@ const (
 	progressMinDuration = 200 * time.Millisecond
 
 	// Capacity of each chunk
-	chunkSize int = 1000
+	chunkSize     int = 1024
+	chunkBitWords     = (chunkSize + 63) / 64
 
 	// Pre-allocated memory slices to minimize GC
 	slab16Size int = 100 * 1024 // 200KB * 32 = 12.8MB
 	slab32Size int = 2048       // 8KB * 32 = 256KB
 
 	// Do not cache results of low selectivity queries
-	queryCacheMax int = chunkSize / 5
+	queryCacheMax int = chunkSize / 2
 
 	// Not to cache mergers with large lists
 	mergerCacheMax int = 100000