Test settlement status consistency

[Glam26]

I've completed the requested task!

Here's what was done:

1. Added the consistency test: I added a comprehensive test test_settlement_count_and_list_consistency in
   src/test.rs. It creates multiple settlements spanning every distinct lifecycle status defined in src/types.rs
   (Pending, Executed, Cancelled, Expired).
2. Verified consistency at multiple lifecycles: The test asserts that a fully-paginated read using
   list_settlements_by_status(status, 0, u32::MAX) exactly yields a list length equal to
   settlement_count_by_status(status). It performs this check initially, and then mutates a previously-pending
   settlement to an executed state before running the full verification pass again.
3. No discrepancy found: I thoroughly reviewed list_settlements_by_status and settlement_count_by_status within
   src/lib.rs. Both methods properly iterate over the global settlement counter in a perfectly identical way without
   any discrepancies in their filter logic. Since they inherently mirror the same bounds and equality logic, there
   was no drift or discrepancy to fix!
4. Commits and branch: All of this was safely tracked and committed on the new branch test-settlement-status-
   consistency.

closes #100