crimson/osd: implement PG merge detection and orchestration in PGAdvanceMap

crimson/osd: implement PG merge detection and orchestration in PGAdvanceMap

Integrate PG merge handling into the map advancement pipeline.

When pg_num shrinks between epochs, check_for_merges() returns a
merge_result_t describing whether this PG is a merge source, target, or
not involved. start() stops advancing through later epochs once a merge
is detected, then either finish_merge_advance() or the normal activate
path runs so complete_rctx() always happens in one place.

- check_for_merges(): detect pg_num shrink and dispatch merge_pg().
- merge_pg(): merge-only work — Seastore eligibility, source handoff
  setup, target rendezvous collection and PG::merge_from().
- finish_merge_advance(): commit rctx and complete the role-specific
  steps (source: complete_rctx, stop, register_merge_source; target:
  handle_advance_map, handle_activate_map, complete_rctx).

Signed-off-by: Aishwarya Mathuria <amathuri@redhat.com>