feat(client): add core single-active multi-resolver failover

[nightowlnerd]

Summary

• add resolver-manager based single-active multi-resolver flow in client runtime
• add startup failover rotation and multi-signal failover guards to reduce false demotion
• keep recursive standby resolvers inactive in steady state while preserving mixed-mode authoritative standby behavior

Notes

• branch intentionally excludes bench-threshold and pacing-experiment commits from prior branch
• current CI blocker is Bench rust-rust-mixed throughput floor 25 MiB/s, which appears threshold-sensitive rather than functional

Validation

• cargo fmt
• cargo clippy -p slipstream-client --all-targets -- -D warnings
• cargo test -p slipstream-client
• canary and soak verification on remote VPS environments with anonymized reporting