feat(protocol): add shared WebSocket contract package

apps/vela-protocol/src/index.d.ts

@@ -0,0 +1,68 @@
+export type SessionState = 'idle' | 'listening' | 'thinking' | 'speaking';
+
+export type MessageEnvelope<TType extends string, TPayload> = {
+	type: TType;
+	payload: TPayload;
+};
+
+export type ClientEventPayloads = {
+	'session.start': Record<string, never>;
+	'input_audio.append': {
+		chunk: string;
+	};
+	'input_audio.commit': Record<string, never>;
+	'response.cancel': Record<string, never>;
+};
+
+export type ServerEventPayloads = {
+	'session.ready': {
+		sessionId: string;
+	};
+	'session.state': {
+		value: SessionState;
+	};
+	'transcript.partial': {
+		text: string;
+	};
+	'transcript.final': {
+		text: string;
+	};
+	'response.text.delta': {
+		text: string;
+	};
+	'response.completed': Record<string, never>;
+	'error': {
+		code: string;
+		message: string;
+		retryable?: boolean;
+	};
+};
+
+export type ClientEventType = keyof ClientEventPayloads;
+export type ServerEventType = keyof ServerEventPayloads;
+
+export type ClientEvent = {
+	[Type in ClientEventType]: MessageEnvelope<Type, ClientEventPayloads[Type]>;
+}[ClientEventType];
+
+export type ServerEvent = {
+	[Type in ServerEventType]: MessageEnvelope<Type, ServerEventPayloads[Type]>;
+}[ServerEventType];
+
+export const PROTOCOL_PACKAGE_NAME: '@vela/protocol';
+export const SESSION_STATES: readonly SessionState[];
+export const CLIENT_EVENT_TYPES: readonly ClientEventType[];
+export const SERVER_EVENT_TYPES: readonly ServerEventType[];
+
+export function createMessageEnvelope<TType extends ClientEventType>(
+	type: TType,
+	payload: ClientEventPayloads[TType]
+): MessageEnvelope<TType, ClientEventPayloads[TType]>;
+export function createMessageEnvelope<TType extends ServerEventType>(
+	type: TType,
+	payload: ServerEventPayloads[TType]
+): MessageEnvelope<TType, ServerEventPayloads[TType]>;
+
+export function isMessageEnvelope(value: unknown): value is MessageEnvelope<string, unknown>;
+export function isClientEventType(type: string): type is ClientEventType;
+export function isServerEventType(type: string): type is ServerEventType;