2cebbd94c4
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