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/handlers/__init__.py

@@ -0,0 +1,24 @@
+# core/handlers/__init__.py
+from __future__ import annotations
+
+from core.handler import EventDispatcher
+from core.protocol import IncomingCallback, IncomingCommand, IncomingMessage
+from core.handlers import callback, chat, message, settings, start
+
+
+def register_all(dispatcher: EventDispatcher) -> None:
+    # Commands
+    dispatcher.register(IncomingCommand, "start",    start.handle_start)
+    dispatcher.register(IncomingCommand, "new",      chat.handle_new_chat)
+    dispatcher.register(IncomingCommand, "rename",   chat.handle_rename)
+    dispatcher.register(IncomingCommand, "archive",  chat.handle_archive)
+    dispatcher.register(IncomingCommand, "chats",    chat.handle_list_chats)
+    dispatcher.register(IncomingCommand, "settings", settings.handle_settings)
+
+    # Messages — catch-all (audio falls back here until voice_handler registered)
+    dispatcher.register(IncomingMessage, "*", message.handle_message)
+
+    # Callbacks
+    dispatcher.register(IncomingCallback, "confirm",      callback.handle_confirm)
+    dispatcher.register(IncomingCallback, "cancel",       callback.handle_cancel)
+    dispatcher.register(IncomingCallback, "toggle_skill", callback.handle_toggle_skill)