Update GitHub Pages landing and demo pages for v0.1.2

- Rename MusicPlugin → MidiPlugin in arch diagram; add CodePlugin as second active plugin (symbol-level OT, 11 languages, tree-sitter AST) - Fallback domain data: "music" → "midi" with 10 named dimensions; add full "code" domain entry with 5 symbol dimensions and .museattributes capability - Ticker: both "midi" and "code" marked active; was just "music" - Protocol section intro updated to list MIDI and code as shipped domains - Ecosystem section lead updated: MIDI + code shipped, rest planned - Footer: v0.1.1 → v0.1.2, add Python 3.14 - Demo page default version fallback: 0.1.1 → 0.1.2