feat: implement core/ and platform/ with full test coverage

- platform/interface.py: PlatformClient Protocol + Pydantic models (User,
  MessageResponse, UserSettings) — no explicit session management, Master
  handles container lifecycle
- platform/mock.py: MockPlatformClient with simulated latency, [MOCK]
  responses, is_new correctly True only on first creation
- core/protocol.py: unified dataclasses for all events and responses
  (IncomingMessage/Command/Callback, OutgoingMessage/UI/Notification,
  AuthFlow, ChatContext, SettingsAction, etc.)
- core/store.py: StateStore Protocol + InMemoryStore (tests) + SQLiteStore
  (prod) with JSON serialization
- core/chat.py: ChatManager — chat metadata (C1/C2/C3), not container
  lifecycle (that's the platform's job)
- core/auth.py: AuthManager — start_flow / confirm / is_authenticated
- core/settings.py: SettingsManager — get/apply with store cache
- core/handler.py: EventDispatcher — registry-based routing with keys
  (command name, action name, attachment type, "*" catch-all)
- core/handlers/: register_all() + start/new/message/callback/settings
  handlers; voice slot falls back to stub text until voice_handler added
- conftest.py: sys.path fix so local platform/ shadows stdlib platform
- docs/api-contract.md: rewritten for Lambda Lab 3.0 container model

46 tests passing, 0 warnings.

core/settings.py

@@ -0,0 +1,29 @@
+# core/settings.py
+from __future__ import annotations
+
+import structlog
+
+from core.protocol import SettingsAction
+from core.store import StateStore
+from platform.interface import PlatformClient, UserSettings
+
+logger = structlog.get_logger(__name__)
+
+
+class SettingsManager:
+    def __init__(self, platform: PlatformClient, store: StateStore) -> None:
+        self._platform = platform
+        self._store = store
+
+    async def get(self, user_id: str) -> UserSettings:
+        cached = await self._store.get(f"settings:{user_id}")
+        if cached:
+            return UserSettings(**cached)
+        settings = await self._platform.get_settings(user_id)
+        await self._store.set(f"settings:{user_id}", settings.model_dump())
+        return settings
+
+    async def apply(self, user_id: str, action: SettingsAction) -> None:
+        await self._platform.update_settings(user_id, action)
+        await self._store.delete(f"settings:{user_id}")  # invalidate cache
+        logger.info("Settings applied", user_id=user_id, action=action.action)