Test flake: office-live-829 "a file change settles into one refresh" fails on a loaded runner #883

What happens: tests/office-live-829.test.ts (added in #832), test "a file change settles into one refresh, one at a time, and an unchanged file never refreshes", sometimes fails in CI with "nothing before the debounce after the last change". Seen in run 43736734435 on #881; that PR doesn't touch office previews, and the test passes locally.

Why: after real delay(10) / delay(20) sleeps, the test asserts that no refresh has happened yet. On a loaded runner a sleep can overrun the debounce window, so the refresh fires early.

Expected: drive the debounce with a fake clock (or a debounce injected by the test) so timing can't fail it, and keep the same assertions.