Clean up: drop orphan CSS in Score + remove test essays

- Score.astro had a dangling declaration block after `.score-stage:active`
  that browsers discarded. Removed; mask-image + --mask-start/--mask-end
  live on `.score-stage` and are set from JS via setFadeMaskVars.
- grid-test.md / score-test.mdx were TEST essays for the BeginSketch
  grid-wave cues and the Score component. Both features shipped; the
  tests no longer earn their list slot on the index.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: RuitingMa

src/components/Score.astro

@@ -96,16 +96,6 @@ const { slug, loop, playhead = 0.22, height = 200 } = Astro.props;
   }
   .score-stage:active {
     cursor: grabbing;
-  }
-    /* Outer horizontal feather so music entering/exiting the stage
-       doesn't snap at the edges. The left fade also softens the
-       overlay's boundary into the body. */
-    -webkit-mask-image: linear-gradient(90deg, transparent 0%, #000 4%, #000 96%, transparent 100%);
-            mask-image: linear-gradient(90deg, transparent 0%, #000 4%, #000 96%, transparent 100%);
-    background: transparent;
-    /* Mask-fade positions — set from JS once header width is known. */
-    --mask-start: 0px;
-    --mask-end: 0px;
   }
   .score-stage:focus-visible {
     outline: 1px solid var(--accent);