Introduce relay-proof lifecycle and per-runtime proxy-generation handling.
Adds endpoint_health_lifecycle.cpp and new relay-proof types/state (RelayProofIdentity/State, relayProofs map) plus Promote/Retire helpers and pruning/TTL. Update policy to prune endpoint state and relay proofs. Forward retire/apply-generation through ProxyControlPlane and SessionProxy adapter and call applyMtproxyProxyGeneration from Instance::migrateProxy. Remove lastRelayAttemptId scalar. Register new source in CMake and bump account limits to 100. Many tests updated to exercise the new lifecycle and generation semantics.
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