Use LSD radix sort for Result sorting in matcher

Replace comparison-based pdqsort with LSD radix sort on the uint64
sort key. Radix sort is O(n) vs O(n log n) and avoids pointer-chasing
cache misses in the comparison function. Sort scratch buffer is reused
across iterations to reduce GC pressure.

Benchmark (single-threaded, Chromium file list):
- linux query (180K matches): ~16% faster
- src query (high match count): ~31% faster
- Rare matches: equivalent (falls back to pdqsort for n < 128)

src/result_x86.go

@@ -14,3 +14,7 @@ func compareRanks(irank Result, jrank Result, tac bool) bool {
 	}
 	return (irank.item.Index() <= jrank.item.Index()) != tac
 }
+
+func sortKey(r *Result) uint64 {
+	return *(*uint64)(unsafe.Pointer(&r.points[0]))
+}