refactor(bench): grade render_bench with the shared frame-timing recorder

render_bench.py (from the parallel perf/render-bench work) had its own
grading module, frame_pacing, with its own definition of a missed frame
and its own refresh estimate. The soak already had both in frame_timing,
so the two could have drifted apart on what "late" means.

The bench now gives the display manager a fresh FrameTimingRecorder,
drains it synchronously at the start and end of the graded run, and prints
frame_soak's report with frame_soak's verdict. Its workload is unchanged:
the synthetic strip, --busy load, the shared speed resolver, the
per-frame scrolling announcement. frame_pacing, its tests and its
src.common exports are removed; measure_refresh_hz moves to frame_timing,
where scroll_speeds.py now finds it.

Two ideas from frame_pacing carry over. The bench seeds the recorder with
the idle refresh it measures, so a loop that free-runs (the 827fps bug
the first bench caught) shows as early frames and one stuck at half rate
as late frames, where an estimate taken from their own intervals finds
both self-consistent. And the soak, which has no idle measurement, now
calls a run NOT LOCKED when its refresh estimate beats the configured cap.
The report also gives the rate held while rendering.

Docs: the bench becomes "Without the service" under "Soaking a rig",
keeping its hdpi numbers and the idle-vs-rendering refresh finding.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/scroll_speeds.py

@@ -42,7 +42,7 @@ from pathlib import Path
 
 sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
 
-from src.common import frame_pacing, scroll_config  # noqa: E402
+from src.common import frame_timing, scroll_config  # noqa: E402
 
 CONFIG = Path(__file__).resolve().parent.parent / "config" / "config.json"
 
@@ -101,11 +101,11 @@ def measure_refresh(config, seconds=6.0):
 
     What an older Pi or a longer chain will really give you, as opposed to
     whatever limit_refresh_rate_hz optimistically asks for. The timing loop
-    itself lives in src.common.frame_pacing so the benchmark grades a soak
-    against the same measurement this ladder is built from.
+    itself lives in src.common.frame_timing so the benchmark grades against
+    the same measurement this ladder is built from.
     """
     matrix = open_matrix(config, refresh_override=0)
-    measured = frame_pacing.measure_refresh_hz(matrix, seconds)
+    measured = frame_timing.measure_refresh_hz(matrix, seconds)
     matrix.Clear()
     return measured