feat(galera): add network queue, PK certification, and split-brain quorum diagnostics #975

Summary of Changes

Add new Galera Cluster diagnostic checks to mysqltuner.pl and comprehensive unit test coverage:

1. Galera Local Send & Recv Queue Monitoring:

   • Detects elevated network queue averages (wsrep_local_send_queue_avg > 0.05 or wsrep_local_recv_queue_avg > 0.05).
   • Recommends checking network bandwidth and storage latency on slower nodes.

2. Galera Primary Key Certification Enforcement Check:

   • Detects when wsrep_certify_non_pk is set to OFF.
   • Recommends enabling wsrep_certify_non_pk = ON to prevent replication inconsistencies and full cluster locks.

3. Galera Split-Brain Quorum Check:

   • Warns when wsrep_cluster_size is an even number (e.g. 2 or 4 nodes) without an arbitrator.
   • Recommends an odd node count (3, 5) or deploying Galera Arbitrator (garbd) to prevent split-brain quorums.

Verification & Unit Tests

• New dedicated unit test file: tests/unit_galera_enhanced.t (3/3 subtests passing).
• Validated via full test suite execution (113 files, 570 subtests passing 100%).
• Validated E2E against a live 3-node MariaDB 11.8.6 Galera Cluster topology.