Add <Score>: embedded Verovio notation with synced playback

Drops MEI-authored music into essays via <Score slug="..." />:

- MEI is rendered by Verovio (WASM, lazy-loaded on first score in view).
  Layer stack: dedicated staff-lines at stage-level (single source, no
  double-stroke), scrolling pan with music, frozen clef/keysig/meter
  overlay pinned to the left, playhead, hint.
- Fixed playhead; pan translates. Notes/chord-symbols/functional labels
  glow via a feathered drop-shadow while sounding (render loop diffs
  `is-active` sets per frame, gated on playing to keep scrub cheap).
- Pre-roll: static rest frame already shows the pre-play pose so the
  first note starts right of the playhead and scrolls in with no jump
  when audio begins.
- Loop mode: 3 SVG copies tile via negative margin-left; translate-wrap
  is invisible; gated against firing during pre-roll.
- Non-loop tail: silent post-audio scroll so the last note walks off
  before reaching the end state.
- Drag to seek (pointer capture + threshold), click to toggle, click at
  end → fade-to-start via opacity transition. backdrop-filter suspended
  while dragging to avoid stutter from per-frame backdrop re-sampling.
- Right-edge entrance: stage mask fades staff lines in first, a wider
  pan-mask fades the notes in later, and a backdrop-filter blur overlay
  sharpens them as they scroll inward.

Deps: @astrojs/mdx (so essays that embed a <Score> can live as .mdx),
      verovio (WASM toolkit, ~1.5MB gzipped, on-demand).

Test content: public/scores/twinkle (no-loop, 8 bars with jazz chord
symbols + functional analysis) and public/scores/ii-V-I (looping 2-bar
cadence). Demo essay: score-test.mdx.

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

Author: RuitingMa

astro.config.mjs

@@ -1,9 +1,12 @@
 // @ts-check
 import { defineConfig } from 'astro/config';
+import mdx from '@astrojs/mdx';
 import fontSubset from './integrations/font-subset.mjs';
 
 // https://astro.build/config
 export default defineConfig({
   site: 'https://sunkenkeep.space',
-  integrations: [fontSubset()],
+  // Order: mdx first so .mdx essays can import components; font-subset runs
+  // at astro:build:done, after everything else has emitted HTML.
+  integrations: [mdx(), fontSubset()],
 });