add property tests for PNCounter merge laws

[GianIac]

If you'd like to help with Numax, this is a great place to start, thank you so much for taking a look !

Extend CRDT property tests with PNCounter merge-law coverage.

Why

• balanced CRDT test coverage improves long-term maintainability
• PNCounter introduces signed behavior that benefits from dedicated invariants
• this is a contained test-only contribution

Scope

• update crates/nx-sync/tests/crdt_properties.rs
• add PNCounter property tests for commutativity, associativity, idempotence
• include roundtrip checks where appropriate

Done when

• cargo test -p nx-sync passes with new PNCounter property coverage
• tests follow existing style and naming conventions
• no unrelated CRDT behavior changes are introduced

Notes

Please keep this issue focused on PNCounter properties only.

Thanks again for considering contributing to Numax.