perf(clean): replace file-based mdfind cache with in-memory lookups (#986)

The orphan scan forked grep twice per candidate to check the mdfind
cache file. On machines with 200+ items in ~/Library/Caches this added
400+ subprocess forks purely for cache hits.

Replace the temp-file cache with newline-delimited bash variables
checked via case glob matching (zero forks, bash 3.2 compatible).
Unify three separate caches into one shared store so bundle IDs
resolved during orphan scanning are not re-queried during system
service or container stub scanning. Add caching to
_container_stub_app_exists which previously had none.

Isolated benchmark: ~15x faster cache lookups (539ms → 33ms for 200
bundle IDs). End-to-end mo clean impact is marginal (~1% of total
runtime) since mdfind queries and filesystem traversal dominate.