fix(sequencer): stop signalling already-executed governance payloads

[spalladino]

Context

A mainnet sequencer kept casting governance signals for a payload whose proposal had already been executed: the outer Multicall3 tx succeeds but the inner signalWithSig reverts (swallowed by allowFailure), wasting ~100k gas per slot. Two client-side bugs allowed this: executed proposals were treated like any other terminal state, so the payload was re-signalled every round; and the TS ProposalState enum was missing Droppable (present in IGovernance.sol since July 2025), so decoding an Expired proposal threw, and the publisher failed open and signalled anyway. Additionally, since the executed payload upgraded the canonical rollup, proposers of the old rollup kept signalling on a GovernanceProposer now keyed to a different instance, where their signals always revert.

Approach

• Fix the ProposalState enum to match IGovernance.sol (9 states including Droppable) and drive the sweep from an explicit LIVE_PROPOSAL_STATES set.
• Replace the boolean hasActiveProposalWithPayload sweep with getPayloadProposalStatus returning 'live' | 'executed' | 'none'. The publisher stops signalling a payload whose proposal was executed, unless the new GOVERNANCE_PROPOSER_FORCE_PAYLOAD_VOTE config is set (L1 permits re-proposing an executed payload, and some payloads are designed to be re-executed). Live proposals take precedence over executed ones, executed verdicts are memoized in-process, and the sweep remains a bounded lookback capped by the protocol-wide proposal lifetime.
• Add a canonicality guard to enqueueCastSignalHelper: resolve the canonical rollup via getRollupAddress() and skip the signal when the configured rollup is not canonical. Only the canonical rollup's current proposer can signal, so a non-canonical node would otherwise waste gas on a reverting signal. Because the guard returns early on a mismatch, round accounting and the EIP-712 digest use the configured (verified-canonical) rollup address.
• The proposal-status guard fails open on RPC errors — at worst a duplicate signal the contract simply counts alongside others in the round — so a flaky L1 endpoint cannot silence governance participation. A failure to resolve the canonical rollup instead skips the signal for that slot and is retried on the next one.

API changes

ReadOnlyGovernanceContract.hasActiveProposalWithPayload and the GovernanceProposerContract wrapper are replaced by getPayloadProposalStatus(payload). The redundant GovernanceProposerContract.getInstance() wrapper is removed in favour of the existing getRollupAddress(). New optional sequencer config governanceProposerForcePayloadVote (env GOVERNANCE_PROPOSER_FORCE_PAYLOAD_VOTE, default false).

To be forward-ported to merge-train/spartan after landing on the v5 line.

Fixes A-1422