refactor: extract magic constants — wave 4 (UX timings + physics)

Name throttle/debounce intervals, spring presets, and layout paddings
that were inline numeric literals:

- VirtualList: VISIBLE_CHANGE_THROTTLE_MS (150), NEAR_BOTTOM_THROTTLE_MS
  (200), JUMP_THROTTLE_MS (200)
- SampleList: CHECK_POSITION_THROTTLE_MS (100)
- SliderArea: SLIDER_SPRING_CONFIG ({stiffness: 0.2, damping: 0.7}),
  SLIDER_PERSIST_DEBOUNCE_MS (100), SLIDER_PADDING_MOBILE_PX (48),
  SLIDER_PADDING_DESKTOP_PX (96)
- FontVirtualList: TOUCH_DEBOUNCE_MS (150)
- createPerspectiveManager: PERSPECTIVE_SPRING_CONFIG ({stiffness: 0.2,
  damping: 0.8})

No behavior changes — values preserved exactly.

src/widgets/SampleList/ui/SampleList/SampleList.svelte

@@ -36,6 +36,12 @@ const SAMPLER_CONTENT_PADDING_X = 32;
 // Matches the previous hardcoded itemHeight={220} value to avoid regressions.
 const SAMPLER_FALLBACK_HEIGHT = 220;
 
+/**
+ * Throttle for the `checkPosition` scroll observer. Trades responsiveness
+ * of the perspective tilt against scroll-handler cost.
+ */
+const CHECK_POSITION_THROTTLE_MS = 100;
+
 let text = $state('The quick brown fox jumps over the lazy dog...');
 let wrapper = $state<HTMLDivElement | null>(null);
 // Binds to the actual window height
@@ -54,7 +60,7 @@ const checkPosition = throttle(() => {
     const viewportMiddle = innerHeight / 2;
 
     isAboveMiddle = rect.top < viewportMiddle;
-}, 100);
+}, CHECK_POSITION_THROTTLE_MS);
 
 // Resolver recreated when typography values change. The returned closure reads
 // fontLifecycleManager.statuses (a SvelteMap) on every call, so any font status