[SC-14] Implement on-chain voting - create proposal and cast vote

[yusuftomilola]

Overview

Implement the on-chain voting system for tokenized asset governance proposals inside contracts/contrib/.

Location

All work must be inside contracts/contrib/src/voting.rs.

Acceptance Criteria

• Proposal struct: proposal_id: u64, asset_id: BytesN<32>, proposal_type: String, yes_votes: i128, no_votes: i128, deadline: u64, min_threshold: i128, status: ProposalStatus (Active, Passed, Rejected, Executed)
• create_proposal(env, asset_id, proposal_type, min_threshold, deadline) -> u64 — returns proposal ID
• cast_vote(env, asset_id, proposal_id, voter, support: bool) — requires auth from voter; voter balance must meet min_threshold; prevents double voting; adds voter balance as vote weight
• Vote weights stored per voter per proposal