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.

tests/core/test_voice_slot.py

@@ -0,0 +1,49 @@
+# tests/core/test_voice_slot.py
+import pytest
+from core.protocol import IncomingMessage, Attachment, OutgoingMessage
+from core.handlers.message import handle_message
+from core.store import InMemoryStore
+from core.auth import AuthManager
+from core.chat import ChatManager
+from core.settings import SettingsManager
+from platform.mock import MockPlatformClient
+
+
+@pytest.fixture
+def deps():
+    platform = MockPlatformClient()
+    store = InMemoryStore()
+    auth_mgr = AuthManager(platform, store)
+    return dict(
+        platform=platform,
+        chat_mgr=ChatManager(platform, store),
+        auth_mgr=auth_mgr,
+        settings_mgr=SettingsManager(platform, store),
+    )
+
+
+async def test_voice_message_returns_stub(deps):
+    await deps["auth_mgr"].confirm("u1")
+    msg = IncomingMessage(
+        user_id="u1", platform="telegram", chat_id="C1", text="",
+        attachments=[Attachment(type="audio", filename="voice.ogg")],
+    )
+    result = await handle_message(event=msg, **deps)
+    assert len(result) == 1
+    assert isinstance(result[0], OutgoingMessage)
+    assert "голосов" in result[0].text.lower()
+
+
+async def test_text_message_calls_platform(deps):
+    await deps["auth_mgr"].confirm("u1")
+    msg = IncomingMessage(user_id="u1", platform="telegram", chat_id="C1", text="Привет!")
+    result = await handle_message(event=msg, **deps)
+    texts = [r.text for r in result if isinstance(r, OutgoingMessage)]
+    assert any("[MOCK]" in t for t in texts)
+
+
+async def test_unauthenticated_user_gets_start_prompt(deps):
+    msg = IncomingMessage(user_id="new_user", platform="telegram", chat_id="C1", text="hello")
+    result = await handle_message(event=msg, **deps)
+    assert len(result) == 1
+    assert "/start" in result[0].text