feat(node): coordinate deferred Tempo imports

[shekhirin]

• Queues finalized L1 blocks until they are imported by the Zone chain.
• Produces bounded checkpoint-only blocks while catching up after T12 activation, without crossing the activation boundary.
• Uses the finalized L1 target during catch-up.
• Uses Tempo T12 activation for checkpoint eligibility and Zone block timestamp behavior.
• Retires queued L1 blocks only after their Zone imports are confirmed.
• Replicates deferred imports consistently to follower nodes.
• Recovers cursors from both legacy and T12 TempoAdvanced events.
• Keeps checkpoint-only blocks leader-neutral so operational leadership changes only take effect in full blocks.