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/protocol.py

@@ -0,0 +1,124 @@
+# core/protocol.py
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+from datetime import datetime
+
+
+@dataclass
+class Attachment:
+    type: str  # "image" | "document" | "audio" | "video"
+    url: str | None = None
+    content: bytes | None = None
+    filename: str | None = None
+    mime_type: str | None = None
+
+
+@dataclass
+class IncomingMessage:
+    user_id: str
+    platform: str
+    chat_id: str
+    text: str
+    attachments: list[Attachment] = field(default_factory=list)
+    reply_to: str | None = None
+
+
+@dataclass
+class IncomingCommand:
+    user_id: str
+    platform: str
+    chat_id: str
+    command: str
+    args: list[str] = field(default_factory=list)
+
+
+@dataclass
+class IncomingCallback:
+    user_id: str
+    platform: str
+    chat_id: str
+    action: str
+    payload: dict = field(default_factory=dict)
+
+
+@dataclass
+class OutgoingMessage:
+    chat_id: str
+    text: str
+    parse_mode: str = "plain"
+    attachments: list[Attachment] = field(default_factory=list)
+    reply_to: str | None = None
+
+
+@dataclass
+class UIButton:
+    label: str
+    action: str
+    payload: dict = field(default_factory=dict)
+    style: str = "secondary"  # "primary" | "danger" | "secondary"
+
+
+@dataclass
+class OutgoingUI:
+    chat_id: str
+    text: str
+    buttons: list[UIButton] = field(default_factory=list)
+
+
+@dataclass
+class OutgoingNotification:
+    chat_id: str
+    text: str
+    level: str = "info"  # "info" | "warning" | "success" | "error"
+
+
+@dataclass
+class OutgoingTyping:
+    chat_id: str
+    is_typing: bool
+
+
+@dataclass
+class ChatContext:
+    chat_id: str
+    display_name: str
+    platform: str
+    surface_ref: str
+    created_at: datetime
+    is_archived: bool = False
+
+
+@dataclass
+class AuthFlow:
+    user_id: str
+    platform: str
+    state: str  # "pending" | "code_sent" | "confirmed" | "failed"
+    platform_user_id: str | None = None
+
+
+@dataclass
+class ConfirmationRequest:
+    action_id: str
+    chat_id: str
+    description: str
+    risk_level: str  # "low" | "medium" | "high"
+    expires_at: datetime
+
+
+@dataclass
+class SettingsAction:
+    action: str
+    payload: dict = field(default_factory=dict)
+
+
+@dataclass
+class PaymentRequired:
+    user_id: str
+    reason: str
+    current_plan: str
+
+
+# Type aliases
+IncomingEvent = IncomingMessage | IncomingCommand | IncomingCallback
+OutgoingEvent = OutgoingMessage | OutgoingUI | OutgoingNotification | OutgoingTyping