feat(rdma): remove IB-degraded nodes from placement ring

[ketor]

Change

• probe every resolved server IB rail on registration/heartbeat
• publish backward-compatible HLT1 member health through MDS
• exclude degraded nodes from kListMembers while retaining them in kListTopology
• add dfkvctl topology device/status display, recovery hysteresis, and Prometheus gauges

Compatibility

Old nodes default eligible. Old clients automatically receive the filtered kListMembers placement view. New dfkvctl falls back when an old MDS does not support kListTopology.

Verification

• local CTest: 689 discovered, all runnable tests passed
• membership/health state tests: 19 PASS
• 0064 two-server real mlx5 topology: 2/2 active; injected one rail Polling -> node-b 0 vnodes and ordinary placement view 1 node while process stayed healthy; three healthy heartbeats -> automatic 2/2 recovery

No environment outside xb01-gpu-200b-0064 was changed.