feat(node): stop Zone block production while the portal is paused

[0xKitsune]

This PR makes a finalized Portal pause stop Zone execution without stopping L1 governance.

The producer and followers share the same pause gate: leaders finish at most the in-flight block, followers retain peer blocks without importing them, and neither side signs settlements while paused.

if drain.production_paused() {
    return Ok(None);
}

Portal state is initialized before startup and tracked independently of the bounded execution queue. RPC failures fail closed, while resume() or automatic expiry clears the gate and drains retained work.

if self.context.l1_block_tracker.portal_paused() {
    return Ok(PeerBlockImportOutcome::Paused(None));
}

Settlement retries also reuse a valid anchor until it approaches expiry, giving delayed follower signatures time to reach quorum while still rebuilding and validating the statement against current L1 state.