docs(config): mark target_fps legacy; describe what Vegas scroll_delay does

- General tab "Scroll Frame Rate" (target_fps) is labelled legacy: after
  the sports_scroll fix nothing in core scrolling reads it. The field and
  its API validation stay so saved configs and plugins that read
  global_config['target_fps'] keep working. CONFIG_REFERENCE says the same.
- Vegas frame_based_scrolling/scroll_delay were described as frame-count
  stepping at ~50 FPS. Neither steps nor sets a frame rate: frame-based
  mode converts the speed to px per scroll_delay, clamps it to 0.1-5, and
  still advances by elapsed time, so the applied speed is
  clamp(scroll_speed * scroll_delay, 0.1, 5) / scroll_delay px/s. The
  config comments, render_pipeline comment and CONFIG_REFERENCE rows now
  say so. No behaviour change.

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

src/vegas_mode/config.py

@@ -155,9 +155,14 @@ class VegasModeConfig:
     target_fps: int = 125  # Target frame rate
     buffer_ahead: int = 2  # Number of plugins to buffer ahead
 
-    # Scroll behavior
+    # Scroll behavior. Neither key steps the scroll or sets a frame rate:
+    # motion is always by elapsed time at scroll_speed px/s. With
+    # frame_based_scrolling the speed is first converted to px per
+    # scroll_delay and clamped to 0.1-5 (ScrollHelper.set_scroll_speed), so
+    # the speed actually applied is clamp(scroll_speed * scroll_delay, 0.1, 5)
+    # / scroll_delay -- at the 0.02 default, speeds under 5 px/s run at 5.
     frame_based_scrolling: bool = True
-    scroll_delay: float = 0.02  # 50 FPS effective scroll updates
+    scroll_delay: float = 0.02  # only feeds the clamp above; not a frame period
 
     # Dynamic duration
     dynamic_duration_enabled: bool = True

src/vegas_mode/render_pipeline.py

@@ -127,12 +127,14 @@ class RenderPipeline:
         self.scroll_helper.set_sub_pixel_scrolling(self.config.smooth_scroll)
 
         # Config scroll_speed is always pixels per second, but ScrollHelper
-        # interprets it differently based on frame_based_scrolling mode:
-        # - Frame-based: pixels per frame step
-        # - Time-based: pixels per second
+        # takes it in different units depending on frame_based_scrolling:
+        # - Frame-based: pixels per scroll_delay seconds (clamped to 0.1-5)
+        # - Time-based: pixels per second (clamped to 1-500)
+        # Both modes then advance by elapsed time; frame-based mode does not
+        # step. So frame-based with scroll_delay only adds the clamp: the
+        # applied speed is clamp(scroll_speed * scroll_delay, 0.1, 5) /
+        # scroll_delay px/s.
         if self.config.frame_based_scrolling:
-            # Convert pixels/second to pixels/frame
-            # pixels_per_frame = pixels_per_second * seconds_per_frame
             pixels_per_frame = self.config.scroll_speed * self.config.scroll_delay
             self.scroll_helper.set_scroll_speed(pixels_per_frame)
         else: