feat: DAO governance system — supply cap, mutable decimals, and Pausable for XCAD

[Copilot]

Adds a full on-chain DAO governance layer to the XCAD ecosystem. BITGOV holders propose and vote on parameter changes; passed proposals execute through a timelock before taking effect. XCAD's owner is now the timelock, so no single account can mint or change parameters unilaterally.

BITGOV — ERC20Votes

• Added ERC20Votes + EIP712 to enable on-chain voting checkpoints
• _update override resolves ERC20Capped × ERC20Votes conflict
• Holders must call delegate(self) once to activate voting weight

XCAD — DAO-controlled parameters

• supplyCap (uint256, 0 = uncapped) enforced in mint(); adjustable via setSupplyCap()
• Mutable decimals via setDecimals(uint8) — cosmetic only, does not rescale stored balances
• Pausable circuit-breaker (pause() / unpause()) blocks all _update calls (transfers, mints, burns)
• All setters are onlyOwner (timelock in production)
• Constructor gains a 4th initialSupplyCap arg (0 for uncapped)

New governance contracts

Contract	Role
XCADTimelockController	Owns XCAD; imposes mandatory delay before executing approved actions
XCADGovernor	OZ Governor stack: GovernorSettings + GovernorCountingSimple + GovernorVotes + GovernorVotesQuorumFraction + GovernorTimelockControl

Example proposal to raise the supply cap:

// Encode the call
bytes memory calldata_ = abi.encodeCall(XCAD.setSupplyCap, (2_000_000 * 10**6));

// Full lifecycle
governor.propose([xcadAddr], [0], [calldata_], "Raise cap to 2M");
// ... voting delay ...
governor.castVote(proposalId, 1 /* For */);
// ... voting period ...
governor.queue([xcadAddr], [0], [calldata_], descriptionHash);
// ... timelock delay ...
governor.execute([xcadAddr], [0], [calldata_], descriptionHash);

Ignition module

ignition/modules/governance/XCADGovernor.ts deploys all four contracts in dependency order and configures PROPOSER_ROLE, CANCELLER_ROLE, and EXECUTOR_ROLE on the timelock. After deployment, the deployer should renounce DEFAULT_ADMIN_ROLE to make governance fully self-sovereign.

Optimizer

Enabled Solidity optimizer (runs: 200) in hardhat.config.ts — XCADGovernor exceeds the 24 576-byte EVM limit without it.