perf(h1): reduce idle validation latency

[ronag]

Summary

• schedule reused-idle-socket validation with setImmediate instead of setTimeout(0)
• cancel the matching handle with clearImmediate
• retain the poll turn that surfaces unsolicited bytes / FIN / RST before the next request, without Node's timer floor

Adapted from nodejs/undici#5499 / issue #5493.

Performance

Five paired sequential keep-alive runs (1,000 measured requests/run):

• baseline p50 median: 1.535 ms
• patched p50 median: 0.123 ms
• p50 latency reduction: 92.0%
• median throughput: ~621 → ~5,668 req/s (+813%)

Validation

• response-queue-poisoning + keep-alive/issue-810 suite: 14/14 pass
• low-concurrency no-timeout full run under heavy host load: 512 unit pass, 6 skip; 162 node integration pass
• npm run lint -- --quiet
• git diff --check

[Copilot]

Pull request overview

This PR reduces HTTP/1 keep-alive reuse validation latency by scheduling the “idle socket validation” step with setImmediate instead of setTimeout(0), avoiding Node’s timer floor while still preserving the extra event-loop turn that can surface unsolicited bytes/FIN/RST before binding the next request.

Changes:

• Switch reused-idle-socket validation scheduling from setTimeout(..., 0) to setImmediate(...).
• Switch cancellation from clearTimeout(...) to clearImmediate(...).
• Keep the existing .unref?.() behavior on the scheduled handle.

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