fix(cache): attempt the companion delta BEFORE the block table is trimmed

The MLLM delta derive landed inert. It ran after trim_block_table, so it
reconstructed attention KV at the block-aligned floor while the companion it
had to pair with sat above that floor, and its own offset guard then correctly
refused the seed. Measured on a growing Qwen3.8 chat: checkpoint 38467, trim to
38464, full hit 38528 -- and zero DELTA lines in the log, with the hit still
discarded for a full prefill.

Only the untrimmed hit can be sliced to the checkpoint. The attempt now runs
first, and the trim is reached only once the delta has declined, so the
existing resume-or-abandon path is unchanged when it does.

Caught by running the reproduction rather than trusting the unit tests, which
exercised the helper directly and so never saw the ordering.