fix(tsconnect): stop reporting a deliberate shutdown as a panic

The exit handler called onExit("Unexpected shutdown") whenever the Go
runtime exited. That was upstream's wording from when nothing could stop
the runtime, so every exit really was a panic. This fork added shutdown(),
so a clean teardown now reports itself as a crash to the panic handler
createIPN() hands to its callers.

Split the two cases. Before the IPN reaches the caller an exit is a
startup failure, and rejecting hands it back as an error rather than as a
side-channel callback. After that the caller holds the only shutdown
path, so report the exit without claiming it was unexpected.

Found in review of webnet/webnet#188.

Co-Authored-By: claude-opus-5 <noreply@anthropic.com>

cmd/tsconnect/src/lib/start-ipn.ts

@@ -27,15 +27,29 @@ export async function startIPN(
   })
 
   go.env[INIT_CALLBACK_ENV] = name
-  const exited = go.run(instance).then(() => {
+
+  // An exit before the IPN reaches the caller is a startup failure, and throwing
+  // hands it back as a rejection. Afterwards the caller holds the only shutdown
+  // path, so an exit is either that shutdown or a panic; report it either way,
+  // because the IPN is dead in both cases.
+  let handedOver = false
+  const exited: Promise<never> = go.run(instance).then(() => {
     delete globals[name]
-    onExit("Unexpected shutdown")
-    throw new Error("Go runtime exited before the IPN was ready")
+    if (handedOver) onExit("Go runtime exited")
+    // Always reject: before the handover this is what fails the race below,
+    // and after it the race has settled, so nothing observes the rejection.
+    throw new Error(
+      handedOver
+        ? "Go runtime exited"
+        : "Go runtime exited before the IPN was ready"
+    )
   })
 
   const [newIPN, terminate] = await Promise.race([ready, exited])
   try {
-    return await newIPN(config)
+    const ipn = await newIPN(config)
+    handedOver = true
+    return ipn
   } catch (err) {
     // Nothing was built, so nothing can shut the runtime down. Exit it here and
     // wait for it, or the page keeps a blocked runtime for a failed startup.