feat(protocol): add shared WebSocket contract package

apps/vela-protocol/src/index.cjs

@@ -0,0 +1,52 @@
+const PROTOCOL_PACKAGE_NAME = '@vela/protocol';
+
+const SESSION_STATES = Object.freeze(['idle', 'listening', 'thinking', 'speaking']);
+
+const CLIENT_EVENT_TYPES = Object.freeze([
+	'session.start',
+	'input_audio.append',
+	'input_audio.commit',
+	'response.cancel'
+]);
+
+const SERVER_EVENT_TYPES = Object.freeze([
+	'session.ready',
+	'session.state',
+	'transcript.partial',
+	'transcript.final',
+	'response.text.delta',
+	'response.completed',
+	'error'
+]);
+
+function createMessageEnvelope(type, payload) {
+	return { type, payload };
+}
+
+function isMessageEnvelope(value) {
+	return Boolean(
+		value &&
+		typeof value === 'object' &&
+		typeof value.type === 'string' &&
+		'payload' in value
+	);
+}
+
+function isClientEventType(type) {
+	return CLIENT_EVENT_TYPES.includes(type);
+}
+
+function isServerEventType(type) {
+	return SERVER_EVENT_TYPES.includes(type);
+}
+
+module.exports = {
+	PROTOCOL_PACKAGE_NAME,
+	SESSION_STATES,
+	CLIENT_EVENT_TYPES,
+	SERVER_EVENT_TYPES,
+	createMessageEnvelope,
+	isMessageEnvelope,
+	isClientEventType,
+	isServerEventType
+};