feat: add 21-dimensional MIDI demo page (Bach BWV 846 × Muse VCS)

Self-contained GitHub Pages demo at midi-demo.html:

- 533 authentic notes from Bach Prelude No. 1 in C Major (BWV 846) sourced from the music21 corpus (public domain, CC0) - Piano roll (D3.js SVG): 4-octave range, 3-voice color coding, scrollable 762px time axis with bar grid, animated playhead - Salamander Grand Piano audio via Tone.js (real piano samples from CDN) - Commit DAG (D3 SVG): 8 commits across 3 branches with edge routing, glow selection, branch color lanes, typewriter command log - 21-dimension panel: per-dimension activity bars with glow, grouped by category (core / expression / CC / effects / meta) - Dimension heatmap: commits × 21 dimensions grid with intensity color - CLI reference: 12 MIDI-specific commands with all flags and return values - Keyboard shortcuts: arrow keys (prev/next commit), space (play/pause) - tools/render_midi_demo.py: typed Python generator (mypy clean) - pages.yml: new "Generate MIDI demo page" step - Landing page: nav link + hero CTA for midi-demo.html