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
2cebbd94c4