Replace deterministic .index.tmp path with per-save unique temp files
via tempfile::Builder. Eliminates race condition where background
indexer thread and TUI main thread could collide on the same temp path,
causing data truncation or corrupt cache writes.
Changes:
- Add write_cache_atomically() helper using tempfile::Builder
- Refactor save_with_hash() and save() to use the helper
- Extract encode_cache() to deduplicate serialization logic
- Move tempfile from [dev-dependencies] to [dependencies]
- Add 2 concurrent tests validating no corruption under parallel writes
Fixes#17
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
Add xxhash-rust and bincode workspace dependencies for fast hashing and serialization. Implement cache_util for cache directory/path resolution with versioning, and IndexCache for saving/loading line indices to disk with file-hash validation.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
