fix: eliminate TOCTOU race in IndexCache::save (closes #5)

spawn_indexer builds LineIndex from mmap snapshot but IndexCache::save()
re-opened the file to compute the hash. If the file changed between those
two steps, the cached index would be stored under the wrong hash.

- Add IndexCache::save_with_hash() that computes hash from in-memory data
- Add compute_data_hash() public function (same algorithm as compute_file_hash)
- Update spawn_indexer, FileReader::save_cache, and test callers

crates/core/src/io/progressive_reader.rs

@@ -314,7 +314,7 @@ pub fn spawn_indexer(
 
         let line_index = LineIndex::from_bytes(data);
 
-        let _ = IndexCache::save(&path, &line_index);
+        let _ = IndexCache::save_with_hash(&path, &line_index, data);
 
         let reader = FileReader::from_parts(path, mmap, line_index);