v0.22.0
feat: add commerce + escrow scheme support
- PaymentPayload, PaymentRequirements, PaymentRequirementsV2 now accept scheme: exact | escrow | commerce
- Aligns with facilitator v1.43.0 which serves 'commerce' as alias for 'escrow'
- Default scheme remains 'exact' — no breaking changes
- 4 new tests for scheme validation