v10.18.0 memoized NLI finder: the nightly sweep stops re-proving old negatives

Added

• nli_scan_cache: the phase-4 finder's line-level score is a pure
  function of the two contents, so it is memoized keyed on content
  hashes. Unchanged pairs cost nothing on later runs; a changed hash
  invalidates exactly that pair. MNEMOS_NLI_FINDER_MAX_PAIRS now
  budgets only NEW scorings, and never-scored pairs backfill across
  subsequent nights. Measured before the cache on the production store:
  ~185 static pairs re-scored for ~31 minutes, nightly; steady state
  after: seconds, proportional to the day's new memories. Dry runs read
  the cache but never write it. Phase 6 drops rows referencing archived
  memories (cleanup_scan_cache, mirroring stale-link cleanup).