COMMIT_MESSAGE.txt

feat(escrow): add milestone approval expiry flow

Implement comprehensive milestone approval system with TTL-based expiry
for secure, time-limited approvals in the TalentTrust escrow contract.

## Features
- Four authorization modes (ClientOnly, ArbiterOnly, ClientAndArbiter, MultiSig)
- TTL-based approval expiry (~7 days) in temporary storage
- Fail-closed design: missing/expired approvals prevent release
- Approval clearing after release prevents reuse
- Comprehensive test suite with 20+ tests

## Security
- Role-based access control enforced at approval and release
- Automatic expiry via Soroban temporary storage TTL
- Arbiter validation prevents role overlap with client/freelancer
- Balance and state checks maintained
- Duplicate approval prevention
- Authorization mode enforcement

## Implementation Details
- MilestoneApprovals stored in temporary storage with PENDING_APPROVAL_TTL_LEDGERS (120,960 ledgers ≈ 7 days)
- Approvals auto-expire via TTL and are treated as absent
- Release requires valid, non-expired approvals based on ReleaseAuthorization mode
- Approvals cleared after successful release to prevent reuse

## Files Added
- contracts/escrow/src/ttl.rs - TTL constants for approval expiry
- contracts/escrow/src/approvals.rs - Core approval logic
- contracts/escrow/src/test/approval_expiry.rs - Comprehensive test suite

## Files Modified
- contracts/escrow/src/types.rs - Added MilestoneApprovals, ReleaseAuthorization, extended Contract and Error
- contracts/escrow/src/lib.rs - Updated contract functions with approval flow
- contracts/escrow/src/test.rs - Added test helpers and module includes
- contracts/escrow/src/test/access_control.rs - Updated to use Error enum
- docs/escrow/milestone-validation.md - Comprehensive approval flow documentation

## Testing
- 20+ integration tests covering all authorization modes
- Unit tests for approval logic
- Edge case coverage (expiry, duplicates, unauthorized, invalid state)
- Multiple milestone approval scenarios

## Security Assumptions
- Fail-closed: missing or expired approvals prevent release
- TTL enforcement via Soroban temporary storage
- Role-based authorization enforced
- Approvals are single-use (cleared after release)
- State machine integrity maintained