feat: implement transfer ownership functionality for funded commitments

[Meet-hybrid]

Title
feat(contracts): add transfer_ownership entrypoint for marketplace trading

Summary
This PR implements a secure transfer_ownership entrypoint in the escrow contract to enable secondary trading of marketplace commitments. It ensures only the current owner can authorize transfers, restricts transfers to funded commitments, and maintains accurate owner indexes for both old and new owners.

What Changed
Added transfer_ownership(commitment_id, new_owner) function in contracts/escrow/src/lib.rs.

Enforced authorization via require_auth() for the current owner.

Restricted transfers to Funded commitments only.

Updated Commitment.owner and maintained OwnerIndex for both addresses.

Added tests in contracts/escrow/src/test.rs covering:

Successful transfers

Unauthorized attempts

Index updates

Non‑Funded commitment edge cases

Documented transfer flow, authorization rules, and Funded‑only restriction in contracts/README.md.

Linked Issue
Closes #470

Type of Change
[x] feat

[x] docs

[ ] fix

[ ] refactor

[x] test

[ ] chore

Validation
[x] Tests passed with ≥95% coverage.

[x] Manual verification of index updates and authorization checks completed.

Documentation
[x] Transfer flow documented in README.md.

Checklist
[x] Branch name uses feat/

[x] Commit messages follow Conventional Commits

[x] PR scope matches issue acceptance criteria

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[Meet-hybrid]

Closes #470