Make display-link tick tests deterministic in CI

docs/reference/frame-rate.md

@@ -108,7 +108,7 @@ Frame pacing: the host's frame step, engine frame limiters, present intervals an
 - **Cause:** the acquire waits for the present interval's vsyncs on the display link (`mtl_link_wait_tick`), counted in 60 Hz vsyncs, so two ticks of a 120 Hz link per frame; display mode's interval is 1. Dividing the link rate by the requested rate once per acquire loses fractional ticks (120 / 90 becomes 1).
 - **Fix:** in display mode above 60 Hz the acquire waits as many link ticks as one tick of the present rate takes (`link_wait` in `hle_nvn.c`, `mtl_link_wait_ticks`): one at 120 Hz. Fractional ticks carry across acquires: at 90 Hz, three frames wait four link ticks (1, 1, 2); a rate or pacing-mode change starts a new phase. Rates at or above the link's rate wait one tick.
 - **Found on:** Batman: Arkham Knight (`0100ACD0163D0000`; its profile and note are in a PR of their own): 119.8 fps in the opening cutscene on an M2 Max.
-- **Check:** `t_link_ticks` (also run in CPU-only CI): with a stand-in for the display link's callback, a 60 Hz vsync waits two ticks of a 120 Hz link and one tick of a 60 Hz one, and a wait in the link's own ticks takes one tick per frame. The real acquire checks display pacing at 60, 75, 90, 120 and 240 Hz, game intervals and frame-step caps, including the 60 and 30 Hz link modes. These checks count ticks; they do not measure a game's frame rate on a screen.
+- **Check:** `t_link_ticks` (also run in CPU-only CI): with a controlled clock and one tick delivered per timed wait, a 60 Hz vsync waits two ticks of a 120 Hz link and one tick of a 60 Hz one, and a wait in the link's own ticks takes one tick per frame. The real acquire checks display pacing at 60, 75, 90, 120 and 240 Hz, game intervals and frame-step caps, including the 60 and 30 Hz link modes. Exact tick counts, already elapsed ticks, spurious wakes and silent-link timeouts are checked without depending on OS scheduling: a sleeping ticker could miss the acquire's wall-clock timeout on CI. These checks do not measure a game's frame rate on a screen.
 
 ### Present snapshots: the game's swapchain texture free at once