fix(dsh): synchronize observation state and manage thumbnail capture

packages/dsh-plugin-browserskill/src/observation-http.ts

@@ -6,7 +6,8 @@
  * observation channel through the documented `webServer` route seam instead:
  *
  *   GET  /bsk-observation/state      → { sessions, available }
- *   GET  /bsk-observation/events     → SSE stream of ObservationEvent
+ *   GET  /bsk-observation/events     → SSE snapshot, then ObservationEvent increments
+ *                                    (?thumbnails=0 for state-only subscribers)
  *   POST /bsk-observation/interrupt  → body {sessionId?} → {interrupted: boolean}
  *   POST /bsk-observation/stop       → body {sessionId} → {stopped: boolean}
  *   GET  /bsk-observation/thumbnail/<attachmentId> → image bytes
@@ -99,6 +100,7 @@ export function registerObservationRoutes(
   if (webServer === undefined) {
     return () => {};
   }
+  const streams = new Set<() => void>();
   const disposers = [
     webServer.register({
       kind: "exact",
@@ -129,14 +131,42 @@ export function registerObservationRoutes(
           "cache-control": "no-cache",
           connection: "keep-alive",
         });
-        const unsubscribe = observation.subscribe((event) => {
-          res.write(`data: ${JSON.stringify(event)}\n\n`);
-        });
-        const heartbeat = setInterval(() => res.write(": heartbeat\n\n"), SSE_HEARTBEAT_MS);
-        res.on("close", () => {
+        const thumbnails =
+          new URL(req.url ?? "/", "http://localhost").searchParams.get("thumbnails") !== "0";
+        let unsubscribe: (() => void) | undefined;
+        let heartbeat: ReturnType<typeof setInterval> | undefined;
+        let closed = false;
+        const close = () => {
+          if (closed) return;
+          closed = true;
           clearInterval(heartbeat);
-          unsubscribe();
-        });
+          unsubscribe?.();
+          streams.delete(close);
+          res.end();
+        };
+        const write = (data: string) => {
+          if (closed) return;
+          try {
+            res.write(data);
+          } catch {
+            close();
+          }
+        };
+        streams.add(close);
+        res.on("close", close);
+        res.on("error", close);
+        try {
+          unsubscribe = observation.subscribe(
+            (event) => {
+              write(`data: ${JSON.stringify(event)}\n\n`);
+            },
+            { thumbnails },
+          );
+          if (closed) unsubscribe();
+          else heartbeat = setInterval(() => write(": heartbeat\n\n"), SSE_HEARTBEAT_MS);
+        } catch {
+          close();
+        }
       },
     }),
     webServer.register({
@@ -228,6 +258,7 @@ export function registerObservationRoutes(
     }),
   ];
   return () => {
+    for (const close of streams) close();
     for (const dispose of disposers) dispose();
   };
 }