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_settings.py

@@ -0,0 +1,32 @@
+# tests/core/test_settings.py
+import pytest
+from core.settings import SettingsManager
+from core.store import InMemoryStore
+from core.protocol import SettingsAction
+from platform.mock import MockPlatformClient
+
+
+@pytest.fixture
+def mgr():
+    return SettingsManager(MockPlatformClient(), InMemoryStore())
+
+
+async def test_get_returns_defaults(mgr):
+    settings = await mgr.get("u1")
+    assert "web-search" in settings.skills
+
+
+async def test_apply_toggle_skill(mgr):
+    action = SettingsAction(action="toggle_skill", payload={"skill": "browser", "enabled": True})
+    await mgr.apply("u1", action)
+    settings = await mgr.get("u1")
+    assert settings.skills.get("browser") is True
+
+
+async def test_apply_invalidates_cache(mgr):
+    s1 = await mgr.get("u1")
+    initial = s1.skills.get("browser", False)
+    action = SettingsAction(action="toggle_skill", payload={"skill": "browser", "enabled": not initial})
+    await mgr.apply("u1", action)
+    s2 = await mgr.get("u1")
+    assert s2.skills.get("browser") == (not initial)