feat(scroll): show which scroll speeds are smooth on this panel (#710)

* feat(scroll): show which scroll speeds are smooth on this panel

The Vegas Scroll Speed slider now says what the panel will do with the
chosen speed and offers the nearest smooth ones to click. Backed by
scroll_config.speed_advice() and GET /api/v3/config/scroll-speed-advice,
which uses the refresh the display measured rather than the cap.

Also stops the default 50 px/s snapping to a stepped 48 px/s (2px every 5
refreshes, 24fps) on a 120Hz panel: the low-fps penalty in solve_crisp()
now loses to 60 or 40 px/s. 100Hz panels are unchanged.

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

* fix(scroll): hint threw before its timer variables existed; count 25-30fps as stepped

The Vegas speed hint called refreshScrollSpeedHint() before the let
declarations it uses, so it never rendered (found on ledpi). And the
solver's low-fps penalty stopped at 25fps, which let a measured 125.7Hz
panel keep a 25.1fps 2px-every-5-refreshes scroll.

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

* test: add the scroll-speed-advice route to the /api/v3 URL map snapshot

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

---------

Co-authored-by: Claude Sonnet 5.5 <noreply@anthropic.com>

docs/SCROLL_PERFORMANCE.md

@@ -73,6 +73,10 @@ Sample ladder for a 100 Hz panel:
  100.0 px/s  (1px every 1 refresh  = 100.0 fps, smooth)
 ```
 
+The Vegas **Scroll Speed** slider in the web UI shows the same thing live: a
+line under it says what your speed will run as on this panel, and links to the
+nearest smooth speeds.
+
 ### How a slow speed stays crisp
 
 `SwapOnVSync(canvas, framerate_fraction)` holds each frame for N panel