perf(rdma): evict cold idle endpoints under pressure

[ketor]

Change

• add a non-blocking process-budget reservation path
• on resource pressure, scan idle data/SG/control pools with SIEVE second-chance semantics
• destroy one cold idle endpoint before retrying reservation
• mark cache hits with one guarded visited bit; keepalive does not make cold endpoints hot
• export cache hit/miss/eviction counters

Budget is returned only after RcEndpoint destruction, preserving the merged resource invariant from PR #306.

Verification

• rdma_resource_budget_test: 4/4 PASS
• RdmaSafety filtered tests: 3 PASS, one hardware-dependent skip

No environment is deployed by this PR.