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_auth.py

@@ -0,0 +1,38 @@
+# tests/core/test_auth.py
+import pytest
+from core.auth import AuthManager
+from core.store import InMemoryStore
+from platform.mock import MockPlatformClient
+
+
+@pytest.fixture
+def mgr():
+    return AuthManager(MockPlatformClient(), InMemoryStore())
+
+
+async def test_not_authenticated_initially(mgr):
+    assert await mgr.is_authenticated("u1") is False
+
+
+async def test_start_flow_returns_pending(mgr):
+    flow = await mgr.start_flow("u1", "telegram")
+    assert flow.state == "pending"
+    assert flow.user_id == "u1"
+
+
+async def test_confirm_sets_confirmed(mgr):
+    await mgr.start_flow("u1", "telegram")
+    flow = await mgr.confirm("u1")
+    assert flow.state == "confirmed"
+
+
+async def test_is_authenticated_after_confirm(mgr):
+    await mgr.start_flow("u1", "telegram")
+    await mgr.confirm("u1")
+    assert await mgr.is_authenticated("u1") is True
+
+
+async def test_confirm_without_start_flow(mgr):
+    flow = await mgr.confirm("new_user")
+    assert flow.state == "confirmed"
+    assert await mgr.is_authenticated("new_user") is True