test: cover cancel_vault refund transfer

[1sgtpepper]

Summary

• add lifecycle coverage for cancel_vault refunding the creator through the real token transfer path
• assert the contract escrow balance returns to its pre-create value and vault status becomes Cancelled
• update README_CANCEL_VAULT.md to remove stale simulated-balance wording

Closes #232

Validation

• git diff --check
• cargo fmt -- --check
• stale simulated-balance wording grep over README_CANCEL_VAULT.md, tests/lifecycle.rs, and src/lib.rs