test: verify release_funds succeeds at exact deadline

[emirykl]

Summary

• Add a dedicated test for the inclusive release_funds deadline boundary.
• Set the ledger timestamp to the persisted vault's exact end_timestamp.
• Verify successful release, destination balance transfer, and Completed status.

Test evidence

• cargo test test_release_funds_at_exact_deadline --locked
• cargo test --locked (67 tests passed)
• cargo build --locked --verbose
• cargo clippy --locked -- -D warnings
• cargo fmt -- --check
• Merge-conflict marker guard
• git diff --check

Fixes #331

Verification

• Focused exact-deadline test passed (1 test)
• Full cargo test --locked passed (67 tests)
• cargo build --locked --verbose passed
• cargo clippy --locked -- -D warnings passed
• cargo fmt -- --check passed
• Merge-conflict marker guard passed
• git diff --check passed

Diagnostics

Recovered command failures: /bin/zsh -lc 'cargo test test_release_funds_at_exact_deadline -- --exact' (101)