Intermittent phantom single-failure in Node/Bun runtime suites

[taras]

Twice now a runtime suite has reported one failure with no identifiable failing test: Node once during #264's battery (2117/2118, nothing greppable, two immediate re-runs clean), Bun once during #273's (1 failure with no (fail) line, three re-runs 2166/0). CI green both times; both diffs doc-or-unrelated. Pattern suggests something genuinely intermittent in the runtime suites, not the diffs.

Tracking issue so observations accumulate here instead of PR bodies. When it next appears: capture the full runner output to a file before re-running, note runtime + version + suite + diff context. No action until it has a name.

[taras]

Another concurrent-only phantom Node failure occurred while verifying PR #415 at commit b82121c on macOS arm64 / Node v26.5.1.

The exact deno task verify battery reported test:node exit 1 after 271.8s, but the verifier correctly printed only the earlier Deno failure in full and supplied no identifiable Node failing test. Without a code change, pnpm test:node passed alone: 2,708 tests, 403 suites, 0 failures in 200.3s.

The same battery had a separately identified Deno browser-bundle cleanup failure, now tracked as #417. This observation retains the Node phantom evidence here rather than conflating it with the named Deno failure.