test(runtime-host): attribute Windows startup stalls

[liugddx]

Summary

Adds bounded structured evidence to Runtime Host election timeout failures and a repeatable Windows fresh-root startup stress entry point.

The diagnostic distinguishes the observed startup cut with the exact Candidate PID/attempt identity and exit state, the final election elapsed time, endpoint-connected observation, fixed observation counters, and the last safe registration summary. Desktop and CLI startup errors include the same structured JSON.

This is diagnostic-only. It does not change the 45-second election deadline, retry/respawn policy, candidate backoff, endpoint behavior, public wire protocol, or existing failure classification.

Refs #3279.

Diagnostic boundary

• Candidate launches are counted by launch-object identity, so a coalesced owned launch remains one process attempt even when election asks repeatedly.
• Quick exits retain exact code/signal evidence; running attempts remain distinguishable from exited/unknown attempts.
• The diagnostic does not add the runtime root path or endpoint value to startup errors.
• The detached Candidate still uses stdio: 'ignore'. This PR does not claim to capture child stderr.
• sawEndpointConnected and the observation counters are election observations plus final elapsed time, not exact endpoint publication/phase timestamps.

Capturing child stderr, adding detailed phase timestamps, or changing startup policy remains deferred until this evidence classifies another real failure.

Stress harness

npm run stress:windows-runtime-host-startup -- <iterations> <parallel> <electionDeadlineMs> <settleTimeoutMs> [keep-failures]

The runner creates an isolated root per iteration, emits NDJSON for every iteration plus a summary, coalesces ownership of the exact Candidate attempt, and settles that attempt before removing the root. A rejected spawn is reported as election failure evidence rather than overriding the iteration in cleanup; resolved-attempt settlement errors still propagate.

Verification

L1/L2 on exact head b2315f21eec3076592d86643d492c6e7f5e70183:

• npm run lint: 2,472 files, 0 errors
• npm run format:check: 1,554 files, 0 changes
• Runtime Host, CLI, and Desktop main build/typecheck passed
• checkout-only CI contracts: 55 passed, 0 failed, 1 privilege-dependent symlink skip
• startup-error + stress tests: 9 passed
• targeted election/coalescing: 3 passed
• owned Candidate quick-exit: 1 passed
• CLI Runtime Host context: 9 passed
• Desktop Runtime Host manager: 22 passed
• git diff --check passed
• independent author-gate review: no unresolved P0-P2 findings

Windows dynamic evidence on this exact head:

• fresh-root stress: 1/1 connected in 5.642 seconds
• exact Candidate exited with code 0 / no signal
• Candidate PID exited and the root created by this run was removed

Local limitation: an unrelated full Runtime Host suite process was already stalled on this machine, so the authoritative merged-base full-suite evidence is core CI rather than another local full-suite run.

L3 on the merged result:

• core CI run 32537529458 passed in 13m01s, including the full Runtime Host suite, Desktop E2E, Storybook, and installed CLI validation
• Dependency audit 32537529491 passed in 41s
• Release Windows check 32537529459 passed in 14m05s, including packaging, packaged release smoke, pinned upgrade/uninstall, and automatic update end to end

Remaining work

• Use a future real occurrence to classify Candidate exit, endpoint observation, or stale registration before proposing a behavior fix.
• Decide separately whether a bounded retry/respawn, endpoint hygiene, or deadline policy change is justified.

Fresh merged-base and packaged Windows CI are green; the PR is ready for review.

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: Codex implemented the bounded diagnostic and stress harness, added focused regression coverage, ran local verification, and used an independent review agent for the author gate. The commit includes a Generated-by: Codex trailer.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No