Sprint 6: Channel name dropdown + multi-instance registry + list_instances MCP tool

fix: replace CARemoteLayerServer with CAContext+CALayerHost for cross-process GUI

Upload Source/Shared/Constants.h

v0.4: CARemoteLayerServer architecture — crash-safe + GUI inside Ableton window

Out-of-process rebuild: Add Shared/Constants.h

Out-of-process rebuild: Add Shared/IPCProtocol.h — message definitions for parent↔helper IPC

Out-of-process rebuild: Add Shared/SharedAudioBuffer.h — lock-free shared memory audio transport