docs: withdraw the target_fps claim for sports_scroll; fix the Vegas speed formula

- SPORTS_UNIFICATION.md still presented honouring global target_fps as
  sports_scroll's added behaviour and its one user-visible gain; note that
  it was withdrawn because it had become a speed multiplier.
- ADVANCED_FEATURES.md gave Vegas scrolling as
  (scroll_speed / target_fps) * elapsed; the real rule is scroll_speed px/s
  by elapsed time, through a 0.1-5 px per scroll_delay clamp when
  frame_based_scrolling is on.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/ADVANCED_FEATURES.md

@@ -433,10 +433,14 @@ Vegas mode consists of four core components working together to provide smooth 1
 - **Frame Rate Control:** Precise timing to maintain 125 FPS
 - **Pre-rendered Content:** Plugins pre-render during update()
 
-**Scroll Speed Calculation:**
+**Scroll Speed Calculation:** motion is by elapsed time; `target_fps` paces
+the render loop, not the speed.
 ```python
-pixels_per_frame = (scroll_speed / target_fps)
-scroll_position += pixels_per_frame * elapsed_time
+# frame_based_scrolling: false
+scroll_position += scroll_speed * elapsed_time            # scroll_speed in px/s
+# frame_based_scrolling: true (the default) -- not stepping, just a clamp
+applied = clamp(scroll_speed * scroll_delay, 0.1, 5) / scroll_delay
+scroll_position += applied * elapsed_time
 ```
 
 #### Component Interactions

docs/SPORTS_UNIFICATION.md

@@ -218,6 +218,14 @@ The one behavior the upstreamed version adds is native
 Part A threaded it through each copy by hand, and this makes that threading
 legacy compatibility rather than the mechanism.
 
+> **Superseded.** Once presentation became frame-locked (#545) the helper
+> steps a fixed whole-pixel amount per presented frame and the panel presents
+> at its own refresh, so honouring `target_fps` only turned it into a speed
+> multiplier (60 doubled a scoreboard's speed, 200 halved it). `sports_scroll`
+> no longer reads it: the crisp-speed ladder uses the panel refresh
+> (`display_manager.refresh_hz`), and speed comes from
+> `scroll_settings.scroll_speed` alone. See `docs/SCROLL_PERFORMANCE.md`.
+
 ## Phases
 
 B0–B3 are merged and shipping in core 3.2.0. Everything that remains is
@@ -464,8 +472,9 @@ After adoption plus the frozen legacy copies it was 10,610; removing the dead
 inline duplication (plugins #252) brought it to roughly 8,620. B6 would take it
 to about 3,300 including the shared core module — some 2,400 fewer than before
 this project started. **Until B6 runs, the adoption is net negative on disk**,
-and its one delivered user-visible gain is that adopted plugins honour the
-global `target_fps` instead of hardcoding ~100 FPS.
+and its one delivered user-visible gain was that adopted plugins honoured the
+global `target_fps` instead of hardcoding ~100 FPS (since withdrawn: see the
+note under the B3 design above).
 
 ### Decision: stop adopting further modules until B6 closes