Document all 6 CRDT primitives and MusicRGA on landing page

- Add RGA and AWMap to _compute_crdt_demos(): live demos with HTML visualizations render when running under Python 3.14 venv - RGA demo: two agents insert notes concurrently; ID-ordered merge produces deterministic [C4, E4, G4] sequence - AWMap demo: agent A sets tempo, agent B sets key_sig; join has both (add-wins — concurrent removes cannot evict new tokens) - Update CRDT section text: "four" → "six battle-tested convergent data structures"; add MusicRGA paragraph explaining voice-aware note ordering (bass→tenor→alto→soprano) for concurrent MIDI edits - MIDI domain card: add "MusicRGA" capability pill and update description to mention voice-aware CRDT ordering