…g retire (#21023)

## Summary
- In `seg retire`, `BuildFiles` can leave background build/merge
goroutines running. If `MergeLoop` runs while one of those is still in
flight, its `mergingFiles` CAS loses to the background merge and
`MergeLoop` returns immediately without doing anything.
- The next MergeLoop is typically skipped because an earlier one is
running. And so the whole merge logic was being skipped.
- Drain background work via `agg.WaitForFiles()` between
`BuildFiles`/prune and `MergeLoop` so the merge step actually runs.

## Test plan
- [ ] `make lint` passes
- [ ] `make erigon` builds
- [ ] Run `erigon seg retire` on a datadir where background build/merge
is active and confirm `MergeLoop` performs merges (previously a no-op)

(cherry picked from commit fa2ab7f)

## Summary
- Filter `.kv` files before slicing in `CheckCommitmentRoot`, so
`onlyCheckLastFile` picks the latest `.kv` instead of whatever
`aggTx.Files` returned last (which was often the `.ef`).
- Default `CHECK_COMMITMENT_ROOT_ONLY_LAST_FILE` and
`CHECK_COMMITMENT_ROOT_ONLY_LAST_FILE_RECOMPUTE` to `false` so the check
now covers all `.kv` files by default.
- Add a discovery log line and a clearer warning when no `.kv` files are
present.
- with #21026 -- recompute
check works fine now; and it is fast enough to do for all kv files

## Test plan
- [ ] `make lint`
- [ ] `make erigon integration`
- [ ] Run `erigon seg integrity --check=CommitmentRoot` against a
datadir and confirm all `.kv` files get checked.

(cherry picked from commit a98a674)