…tire (#21025)

Wire the existing `--erigondb.domain.steps-in-frozen-file` flag into two
more entry points so the domain-only merge cap can be overridden when
running these tools directly:

- **`integration stage_exec`** — registers the flag and applies it via
`AggOpts.ErigondbDomainStepsInFrozenFile` when constructing the
aggregator singleton.
- **`erigon seg retire`** — registers the flag on the `seg retire`
subcommand and applies it via a new
`Aggregator.SetErigondbDomainStepsInFrozenFile` runtime setter (the
aggregator is constructed via `openAgg` here, so the override is applied
post-construction, before any merge work begins).

History/inverted-index merges are unaffected — the flag still only
adjusts the domain merge cap, matching the main `erigon` binary's
behavior.

Both commits cherry-picked from the `explores_3` exploration branch.

- [x] `make erigon integration` builds clean
- [x] `make lint` reports 0 issues
- [ ] Manual smoke: run `integration stage_exec
--erigondb.domain.steps-in-frozen-file=Inf` and `erigon seg retire
--erigondb.domain.steps-in-frozen-file=64` against a test datadir,
confirm the override-log line fires and merges respect the new cap

(cherry picked from commit ec7a361)