#116 perf: hand out the cached dataset on an empty search

clearing a search reuses the sorted order already worked out instead of rechecking every row

---

Summary by cubic

Clearing a search now reuses the cached dataset instead of recopying every row, so the sort cache can skip its subset walk on an empty term.

• getCities and getFilms now return the frozen cached array for the empty search term instead of a copy.
• loadEnvelope freezes the cached array, since callers now share it and a mutation would reorder every later caller's rows.
• Public result types are now readonly to reflect that callers don't own a mutable array.