test: add multi-vault isolation coverage

[dangelo352]

Closes #229

Summary

• add a deterministic multi-vault integration test covering three independent vault lifecycles
• drive separate vaults to Completed, Failed, and Cancelled
• assert sequential vault IDs, vault_count, final statuses, and exact balance destinations for each vault
• document the multi-vault isolation scenario in TESTING_GUIDE.md
• include stable rustfmt cleanup for tests/proptest_timestamps.rs that CI expects

Validation

• CARGO_TARGET_DIR=/tmp/disciplr-multivault-target cargo test test_multi_vault_lifecycle_isolation_for_ids_states_and_balances
• CARGO_TARGET_DIR=/tmp/disciplr-multivault-target cargo test — all tests passed
• git diff --check

[1nonlypiece]

appreciate the effort on this — but #229 isn't assigned to you, and we ask contributors to get assigned to an issue before raising a PR. closing as invalid; please claim #229 on the board first, then open it again and we'll take a proper look. https://discord.gg/xvNAvMJf