Per-proposal vote-signature nonce

[LeonardoVieira1630]

Summary

Scopes the EIP-712 vote-signature nonce from account-global to (proposalId, voter): an applied cast on proposal A now invalidates the voter's outstanding signed ballots for proposal A only — held signatures for other open proposals stay valid. Closes the documented UX cost of the account-global design (one direct vote killed a delegate's pending ballots across every open proposal).

Design

• Storage/API: _voteNonces[proposalId][voter] + public voteNonce(uint256,address) — the source of truth for building ballots. The inherited OZ account-global nonces(address) is orphaned at 0 (can't be removed from the inheritance; documented in README).
• Validation reads, _castVote spends (DR-2/DR-4): _validateVoteSig/_validateExtendedVoteSig are overridden as view, building the digest from the per-proposal nonce via the virtual getter. The single spend point is _castVote, before super._castVote dispatches to the ruleset — every cast path (direct, bySig, extended bySig, batch item) converges there, so the invariant voteNonce == applied casts holds by construction and the in-flight signature is dead during the ruleset's external countVote.
• OZ typehashes kept (DR-1): only the nonce source changes. OZ-standard tooling that reads nonces(voter) still produces valid signatures for a voter's first cast on each proposal (both counters start at 0); stale re-vote ballots revert with GovernorInvalidSignature, same as before.
• Removals: the three castVote* overrides and the batch's _useNonce existed only to spend the account-global nonce — all deleted; the contract shrinks.
• No new events/errors (DR-3): VoteCast already tells relayers which (proposal, voter) was invalidated.

Testing

• New suite GovernorNexus.voteNonce.t.sol (9 tests): cross-proposal independence (the target case), same-proposal invalidation, per-path nonce accounting incl. batch duplicates, orphaned nonces() regression guard, fresh re-sign after direct vote, extended-ballot path (previously zero coverage repo-wide), and ERC-1271 contract-signer validate + replay-reject through the new validators.
• Flipped the two tests that encoded the account-global behavior (lifecycle cross-proposal, batch); the batch test now asserts both halves (batched ballot dies, unbatched survives).
• Full suite green: 307/307 unit + 19/19 fork.
• Reviewed: per-task spec+quality reviews, whole-branch security review (replay, cross-proposal/cross-voter digest binding, reentrancy incl. ERC-1271 staticcall), and a scoped re-review of the hardening commit.

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