The local library index used tokio::fs::write directly on the canonical
library_index.json path. That truncates the existing index before writing the
new bytes, so a crash or power loss could leave a zero-length or partial cache.
Write the index through a sibling temp file, sync it, rename it over the
canonical path, and sync the parent directory on Unix. Loading the index also
sweeps a stale temp file before parsing the canonical file. That keeps the
existing cache valid after an interrupted write while still letting a normal
scan rebuild from disk if the canonical index is missing or corrupt.
This follows the existing temp-plus-rename pattern used for version.ini and
install intents. It intentionally does not add locking; local library writes
are already serialized by the peer operation flow.
Test Plan:
just fmt
just test
just clippy
Refs: none
ddidderr
fdad162240
