The Files page's cold-view timing test stops flaking: its ceiling comes from a measured median, not one run #4096

Pull #4020 found that the cold-view test's 250 ms ceiling on app passed or failed depending on the machine (it measured about 290 ms), which is what parked #3418.

Done:

• [ ] The cold-view test in test/pull-timing.test.ts takes the median of 3 cold renders (a fresh head each) and asserts a ceiling written next to the measurement it came from.
• [ ] Run the file 10 times in a row with bun test test/pull-timing.test.ts; it passes every time, and the pull says so.
• [ ] No product code changes.

Files: test/pull-timing.test.ts. Risk: loosening the ceiling until it means nothing. Keep it within 2× the measured median.

Both this and #4095 edit test/pull-timing.test.ts, in different tests: whichever lands second rebases.

Parked by the crew: the run ended without a pull: declined: the cold-view test this issue edits isn't on main, so there is nothing to change here. test/pull-timing.test.ts on this branch has only the Server-Timing phase-names test, and no cold-view timing