feat(governance): add multi-choice proposal support

[devEunicee]

Summary

Closes #298

Extends the governance contract to support proposals with more than three response types. Voters can now choose from any named list of options using a first-past-the-post outcome model.

Changes

contracts/governance/src/types.rs

• Add Vote::Choice(u32) variant — carries the 0-based choice index
• Add ContractError::InvalidChoice = 28
• Add choices: Vec<String> and winning_choice: Option<u32> fields to Proposal

contracts/governance/src/storage.rs

• Add PersistentKey::ChoiceVotes(proposal_id, index) for per-choice vote tallies
• Add choice_votes / set_choice_votes accessors

contracts/governance/src/lib.rs

• create_multi_choice_proposal() — creates a proposal with 2–10 named choices
• get_choice_votes() — query accumulated weight for any choice index
• cast_vote() — routes Vote::Choice to per-index tally; rejects Yes/No/Abstain on multi-choice proposals (and vice-versa)
• finalise() — multi-choice uses first-past-the-post; quorum check on total participation; sets winning_choice

contracts/governance/src/test.rs

• 5 new tests: passing proposal with correct winner, quorum failure, invalid vote type rejection, out-of-bounds index, minimum choices validation

Backward Compatibility

Standard Yes/No/Abstain proposals are unaffected. New choices field defaults to empty Vec for existing proposals.