Bug Fixes — LNVPS Additions

LNVPS-Specific Overrides

Test placement: use the existing #[cfg(test)] mod tests block in the relevant file, or a dedicated tests.rs sibling file.
Example names: test_vm_cost_overflow_with_zero_disk, test_payment_amount_rounds_correctly.
Run tests with: cargo test -- --test-threads=1 (see build-and-test.md)