bug: scenes are Python-side only — not shared between REPL and MCP clients

[jonkhler]

Scenes saved in the REPL (`kr.save("verse")`) are not visible to the MCP server (`recall("verse")` → "scene not found"). Each client has its own Mixer instance with its own `_scenes` dict.

This defeats the purpose of having both REPL and MCP connected simultaneously — Claude Code can't recall scenes the musician saved, and vice versa.

Root cause: Scenes are stored in Python `Mixer._scenes` (client-side), not in the Rust engine (server-side). Each client has its own Mixer.

Options:

1. Store scenes in the engine via IPC (SaveScene/ListScenes/RecallScene commands)
2. Share scenes via a file-based store (~/.krach/scenes/)
3. Sync scenes via the existing Status/Pull mechanism

Option 1 is the clean solution — scenes become engine state, accessible to all clients.

Related: #12-15 (recall/rebuild issues), engine state sync design