setIndexJitterEnabled

Turn jitter on or off for the rest of this process; null restores the default resolution above.

Tests of the jitter *itself* have to force it on — under a test runner it is off, which is the whole point — so they call this and restore afterwards.

``ts beforeAll(() => setIndexJitterEnabled(true)) afterAll(() => setIndexJitterEnabled(null)) ``

Process-wide rather than per-call because the helpers are free functions reached from everywhere, including inside Editor; threading an option through every call site would leave the ones that matter untouched.