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/providers/tts/mock.py

@@ -0,0 +1,49 @@
+"""TTS service implementations used by the engine runtime."""
+
+import asyncio
+from typing import AsyncIterator
+
+from loguru import logger
+
+from providers.common.base import BaseTTSService, TTSChunk, ServiceState
+
+
+class MockTTSService(BaseTTSService):
+    """Mock TTS service for tests and no-provider fallback."""
+
+    def __init__(
+        self,
+        voice: str = "mock",
+        sample_rate: int = 16000,
+        speed: float = 1.0,
+    ):
+        super().__init__(voice=voice, sample_rate=sample_rate, speed=speed)
+
+    async def connect(self) -> None:
+        self.state = ServiceState.CONNECTED
+        logger.info("Mock TTS service connected")
+
+    async def disconnect(self) -> None:
+        self.state = ServiceState.DISCONNECTED
+        logger.info("Mock TTS service disconnected")
+
+    async def synthesize(self, text: str) -> bytes:
+        """Generate silence based on text length."""
+        word_count = len(text.split())
+        duration_ms = word_count * 100
+        samples = int(self.sample_rate * duration_ms / 1000)
+        return bytes(samples * 2)
+
+    async def synthesize_stream(self, text: str) -> AsyncIterator[TTSChunk]:
+        """Generate silence chunks to emulate streaming synthesis."""
+        audio = await self.synthesize(text)
+
+        chunk_size = self.sample_rate * 2 // 10
+        for i in range(0, len(audio), chunk_size):
+            chunk_data = audio[i : i + chunk_size]
+            yield TTSChunk(
+                audio=chunk_data,
+                sample_rate=self.sample_rate,
+                is_final=(i + chunk_size >= len(audio)),
+            )
+            await asyncio.sleep(0.05)