fix: rename tour-de-force demo files to demo-{domain}

tour-de-force-music.md → demo-midi.md tour-de-force-code.md → demo-code.md tour-de-force-script.md → demo-script.md

Updated all references in docs/demo/README.md, docs/README.md, docs/reference/code-domain.md, and the cross-links at the bottom of each demo file. Also corrected the stale "v2 · Python 3.11 · music" footer in docs/demo/README.md to "v0.1.2 · Python 3.13 · MIDI".