add property tests for GCounter 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 GCounter merge-law coverage.

Why

• property tests currently emphasize some CRDT families but not all equally
• GCounter is foundational and ideal for newcomer-friendly CRDT testing
• this improves correctness confidence with focused test work

Scope

• update crates/nx-sync/tests/crdt_properties.rs
• add property tests for GCounter covering commutativity, associativity, idempotence
• include serialization roundtrip checks if consistent with existing patterns

Done when

• cargo test -p nx-sync passes with new GCounter property coverage
• tests are deterministic and follow current test style
• no production-code behavior changes are needed

Notes

Please keep generators small and execution time reasonable.

Thanks again for considering contributing to Numax.