Add SSE event ack support to prevent channel backup

- Track lastHeardEventId and lastAcknowledgedEventId
- Send ack every 20 events (threshold recommended by Urbit)
- Parse event ID from SSE 'id:' lines
- This should fix dropped messages issue

extensions/tlon/src/urbit/sse-client.ts

@@ -43,6 +43,11 @@ export class UrbitSSEClient {
   isConnected = false;
   logger: UrbitSseLogger;
   
+  // Event ack tracking - must ack every ~50 events to keep channel healthy
+  private lastHeardEventId = -1;
+  private lastAcknowledgedEventId = -1;
+  private readonly ackThreshold = 20;
+
   constructor(url: string, cookie: string, options: UrbitSseOptions = {}) {
     this.url = url;
     this.cookie = cookie.split(";")[0];
@@ -214,15 +219,31 @@ export class UrbitSSEClient {
   processEvent(eventData: string) {
     const lines = eventData.split("\n");
     let data: string | null = null;
+    let eventId: number | null = null;
 
     for (const line of lines) {
       if (line.startsWith("data: ")) {
         data = line.substring(6);
       }
+      if (line.startsWith("id: ")) {
+        eventId = parseInt(line.substring(4), 10);
+      }
     }
 
     if (!data) return;
 
+    // Track event ID and send ack if needed
+    if (eventId !== null && !isNaN(eventId)) {
+      if (eventId > this.lastHeardEventId) {
+        this.lastHeardEventId = eventId;
+        if (eventId - this.lastAcknowledgedEventId > this.ackThreshold) {
+          this.ack(eventId).catch((err) => {
+            this.logger.error?.(`Failed to ack event ${eventId}: ${String(err)}`);
+          });
+        }
+      }
+    }
+
     try {
       const parsed = JSON.parse(data) as { id?: number; json?: unknown; response?: string };
 
@@ -249,6 +270,30 @@ export class UrbitSSEClient {
     }
   }
   
+  private async ack(eventId: number): Promise<void> {
+    this.lastAcknowledgedEventId = eventId;
+    
+    const ackData = {
+      action: "ack",
+      "event-id": eventId,
+    };
+    
+    const response = await fetch(this.channelUrl, {
+      method: "PUT",
+      headers: {
+        "Content-Type": "application/json",
+        Cookie: this.cookie,
+      },
+      body: JSON.stringify([ackData]),
+    });
+    
+    if (!response.ok && response.status !== 204) {
+      throw new Error(`Ack failed: ${response.status}`);
+    }
+    
+    this.logger.log?.(`[SSE] Acked event ${eventId}`);
+  }
+
   async poke(params: { app: string; mark: string; json: unknown }) {
     const pokeId = Date.now();
     const pokeData = {