A pull's Files page answers a repeat view of a 40-file diff under 100 ms, and a test holds it #4087

Part two of #4043.

Done:

• [ ] With the per-head cache in place, the repeat view of the 40-file seed in test/pull-timing.test.ts has app under 100 ms, taken as the median of 5 views so a loaded check machine does not flake.
• [ ] The cold ceiling is tightened to what the test measures with headroom, written next to the number with the measurement it came from.
• [ ] If 100 ms is not reachable, the pull names the slowest phase from Server-Timing and stops there rather than loosening the test.