perf: reuse external conformance stack

[lucarlig]

Summary

• reuse the running external dataplane stack for client conformance after the server lane
• retain the existing teardown path on interruption or setup failure
• validate client scenarios before retaining runtime state
• release the behavior as the 0.2.1 patch

Verification

• cargo fmt --all --check
• cargo clippy --all-targets --locked -- -D warnings
• cargo test --all-targets --locked (367 passed)
• cargo package --locked --allow-dirty
• actionlint
• dataplane make conformance using the locally built CLI: 136.35s, down from 364.88s; the known strict-modern registration failure remained visible and returned nonzero

Dataplane build-cache companion: contextforge-org/contextforge-data-plane#127