feat(settlement-workflow): implement on-chain compliance-gated execution workflow

[Markadrian6399]

Summary

• Adds a new contracts/settlement-workflow/ crate implementing the SettlementWorkflow role documented in ARCHITECTURE.md's sequence diagram: gates Treasury::execute_settlement behind Compliance::is_allowed, since Treasury does not consult compliance itself.
• Consumes compliance-client for the compliance check and treasury's generated client directly for execution, per the issue's suggested execution.
• Adds contracts/settlement-workflow/tests/settlement_workflow_test.rs covering: execution blocked when compliance returns false (WorkflowError::ComplianceFailed, no token transfer), and the successful path executing the treasury settlement and transferring tokens to the merchant.
• Registers the new crate in the workspace Cargo.toml.

Closes #274

Validation performed

• cargo fmt -p comebackhere-settlement-workflow passes.
• cargo test --all could not be run: the pre-existing comebackhere-treasury/comebackhere-invoice crates fail to compile in this environment with TreasuryContractArgs/TreasuryContractClient not found errors. This reproduces identically on a clean main checkout with zero changes applied (unrelated to this change, not addressed here per scope). The new contract's logic mirrors the already-tested execute_with_compliance reference implementation in contracts/treasury/tests/compliance_gate_integration_test.rs, calling the same client methods with the same signatures.

[drips-wave]

@Markadrian6399 Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits