refactor: reorder Domain Registry page sections into narrative arc

Old order: Protocol → Diff Algebra → Capabilities → Build → Code → Registry → Ecosystem → Distribution → MuseHub

New order: Protocol → Registry → Ecosystem → Capabilities → Diff Algebra → Build → Code → Distribution → MuseHub

Narrative logic: 1. Protocol — the pitch: 6 methods, here's the contract 2. Registry — proof: it works today (music domain live) 3. Ecosystem — vision: here's where it's going 4. Capabilities — what you get for free (Typed Deltas, OT, CRDT) 5. Diff Algebra — deep dive for the curious (how it works inside) 6. Build — call to action: build your own in 3 steps 7. Code — scaffold detail 8. Distribution — how to share what you build 9. MuseHub — ecosystem endgame