Function save_to_file 

pub fn save_to_file(
    cache: &RuleLevelCache,
    identity: SnapshotIdentity,
    path: &Path,
    max_entries: usize,
) -> Result<usize, SnapshotError>

Write cache’s entries to path, atomically (temp file + rename) so a concurrent reader never observes a half-written snapshot.

At most max_entries are written, taken from the MOST-recently-used end of the cache’s LRU order. The rule cache is bounded by a memory budget (~1/3 of a GiB by default), and a snapshot that large has no business sitting next to a checkpoint: measured mask footprint is ~19 KB/mask at a 151,669-token vocabulary, so the cap is what keeps the file in the tens of megabytes.

Returns the number of entries written.