Timeout/budget stops now return the best compression found so far
instead of discarding all work. Cancellation still returns an error.

- Add CompressResult<W> wrapper with fully_optimized() flag
- lz77_optimal seeds outputstore with greedy result before squeeze loop
- Propagate fully_optimized flag through deflate_part → blocksplit_attempt → encoder
- finish() returns Result<CompressResult<W>, Error> on all three encoders