Refactor project structure and enhance backend integration

- Expanded package inclusion in `pyproject.toml` to support new modules.
- Introduced new `adapters` and `protocol` packages for better organization.
- Added backend adapter implementations for control plane integration.
- Updated main application imports to reflect new package structure.
- Removed deprecated core components and adjusted documentation accordingly.
- Enhanced architecture documentation to clarify the new runtime and integration layers.

engine/runtime/pipeline/constants.py

@@ -0,0 +1,6 @@
+"""Shared constants for the runtime duplex pipeline."""
+
+TRACK_AUDIO_IN = "audio_in"
+TRACK_AUDIO_OUT = "audio_out"
+TRACK_CONTROL = "control"
+PCM_FRAME_BYTES = 640  # 16k mono pcm_s16le, 20ms