The sitemap timing test is a coin flip under load #6194

The sitemap timing test no longer asserts a raw 15 ms. test/sitemap.test.ts now passes locally (2 pass, 0 fail). I ran only that file, not the type check, and haven't seen the forge's check verdict on the push.

• Best of five: the test now clears the listing cache and rebuilds /sitemap.xml five times, keeping the lowest summed Server-Timing phase time.
• Budget helper: that best time goes through expectBudget from test/helpers/budget.ts with a 15 ms budget. The baseline is baselineMs, a /healthz measurement taken in the same run. Under load the limit stretches with the baseline, up to at most 60 ms. With FORGE_TEST_QUIET=1 the absolute 15 ms applies.
• Unchanged: the content checks (100 public repos, no private ones) and the presence of the repos, render and etag timing phases are as before.
• Left out: I didn't touch the ETag/304 test, and I didn't check other tests for raw timing assertions.

To try it, run bun test test/sitemap.test.ts.

Closes #6193