consensus: add fork-gated empty-active-set epoch escape to prevent co…

[finalis-core]

consensus: add fork-gated empty-active-set epoch escape to prevent committee deadlock

• add network fork flag empty_active_set_epoch_escape_activation_height (mainnet: 10145)
• implement deterministic checkpoint fallback in canonical derivation:
  • when next-epoch committee would be empty and active_validator_count == 0,
    select lexicographically smallest bonded non-banned/non-onboarding validator
    as a single-member fallback committee
• mirror identical fallback logic in runtime checkpoint builder to preserve
  canonical/runtime parity and avoid checkpoint recomputation mismatches
• emit runtime trace log:
  • epoch-empty-active-escape ... selected=<pub> reason=deterministic-fallback-member
• keeps consensus progressing across epoch boundaries instead of stalling on
  empty active-set committee collapse