test(plugins): benchmark cold-start and defer trust groups

[MerhiOPS]

Summary

• add a reproducible plugin cold-start benchmark harness for the real volt-plugin-host release binary
• document the benchmark method and current machine results
• defer trust-group shared-process pooling because the measured startup cost is well below the Session 6 thresholds

Benchmark Result

Measured with cargo build -p volt-plugin-host --release and node scripts/ci/plugin-startup-bench.mjs on March 15, 2026.

• Spawn -> ready: min 9.79 ms, median 15.48 ms, max 16.25 ms, p95 16.25 ms
• Spawn -> first command response: min 13.97 ms, median 17.42 ms, max 18.08 ms, p95 18.08 ms
• Decision gate: trust groups are only warranted if spawn -> ready exceeds 200 ms or spawn -> first response exceeds 500 ms
• Decision: do not implement trust groups yet

Validation

• node scripts/ci/plugin-startup-bench.mjs
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• pnpm typecheck
• pnpm test