"/system over server-sent events > the sampler's clock runs only while someone watches, and ticks while they do" passes every time on a loaded check machine #5473

Done:

• [ ] bun test --rerun-each 30 test/system-stream.test.ts -t "the sampler's clock runs only while someone watches" passes, and the pull names the cause it found.
• [ ] Nothing the test promises is weakened: no longer timeouts, no retries, no dropped assertion. Drive the sampler with an injected clock, or wait on its tick event, never a sleep.
• [ ] If the cause is in the product (the clock not stopping when the last watcher leaves under load, say), fix it and say so.

Files: test/system-stream.test.ts and the module the cause is in. Why now: it sank one of #5428's two tries (pull #5437), which only added an MCP tool.