[SC-18] Implement detokenization proposal and voting

[yusuftomilola]

Overview

Implement the detokenization proposal and voting system inside contracts/contrib/.

Location

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

Acceptance Criteria

• DetokenizationProposal struct: proposal_id, asset_id, proposer, deadline, status: DetokenizationStatus (Active, Executed, Rejected)
• propose_detokenization(env, asset_id, deadline) — any token holder may propose; requires minimum token balance; creates proposal via voting contract; emits proposal event
• vote_on_detokenization(env, asset_id, proposal_id, voter, support) — delegates to voting contract cast_vote
• get_detokenization_proposal(env, asset_id) -> DetokenizationProposal — returns active proposal