push updated state to clients over SSE (ISS-129)

lib/sse.ts

@@ -0,0 +1,29 @@
+// ISS-129 (v0.57.0): in-process fan-out registry for the SSE state stream (/api/events). The app
+// runs as one long-lived server process (`next start`; instrumentation.ts already keeps background
+// sim state alive) — no new dependencies, no WS upgrade needed. The Set lives on globalThis because
+// Next.js gives instrumentation.ts (where broadcast() fires from the background tick) and route
+// handlers (where subscribe() happens in /api/events) separate module instances; a plain `let` would
+// give each its own empty set and no push would ever be delivered.
+
+type Push = (text: string) => void;
+
+const g = globalThis as unknown as { __flightSseSubscribers?: Set<Push> };
+const subscribers: Set<Push> = (g.__flightSseSubscribers ??= new Set());
+
+export function subscribe(fn: Push): () => void {
+  subscribers.add(fn);
+  return () => {
+    subscribers.delete(fn);
+  };
+}
+
+// Call each subscriber safely — one dead stream must not take the others down.
+export function broadcast(text: string): void {
+  for (const fn of [...subscribers]) {
+    try {
+      fn(text);
+    } catch {
+      // subscriber's stream is gone; its own cleanup unsubscribes it
+    }
+  }
+}