"anonymous spawn > a signed-in spawn still works (regression)" passes every time on a loaded check machine #5209

Intent: anonymous spawn > a signed-in spawn still works (regression) in test/spawn-anon.test.ts goes red in pulls that never touch it (pull #5170), so good work is parked and the board shows red noise.

Done:

• [ ] the pull names the cause (wall clock, a real sleep, a shared port or directory, a fake clock mixed with Date.now(), a budget measured end to end instead of in the code it guards)
• [ ] bun test --rerun-each 30 test/spawn-anon.test.ts passes, and also passes with other test files running beside it (bun test test/spawn-anon.test.ts test/apps.test.ts). The pull says how it was checked
• [ ] the test still fails when the behaviour it guards is broken; what it promises about the product is unchanged. A time or resource budget may only move to what the code itself spends, never just get wider

Files: test/spawn-anon.test.ts; the source only if the flake is a real race there (then the pull says so). Hint: the signed-in spawn may share a rate-limit window or directory with the anonymous tests before it; give it its own.