"a quiet writer costs almost no CPU while the reader waits" passes every time on a loaded check machine #5208

Intent: a quiet writer costs almost no CPU while the reader waits in test/pull-pipe.test.ts goes red in pulls that never touch it (pull #5131), 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/pull-pipe.test.ts passes, and also passes with other test files running beside it (bun test test/pull-pipe.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/pull-pipe.test.ts; the source only if the flake is a real race there (then the pull says so). Hint: it asserts under 100 ms of process CPU (line 77) across the whole process, which other work on a busy machine inflates; measure only the pipe's own work, or count wakeups instead of CPU time.