sui_v1.79.0_1786777384_ci: [consensus] wire FlexCommitter into Core (#27505)

Description Follow-up to #26876 . When enable_v3 is on, Core uses FlexCommitter for the local commit decision rule ( try_commit_v3 ) and for building sub-dags from certified commits synced by commit syncer ( process_certified_commits ), instead of the UniversalCommitter path. No behavior change while enable_v3 is off (the default). Additional changes on the v3 path: Report per-commit metrics and…