fix(safesocket/js): use unique memconn name per IPN instance

Each call to newIPN() starts an independent Go backend that calls
safesocket.Listen() to serve the ipnserver IPC channel. Because
memName was a global constant, the second instance would fail with
"addr unavailable" and log.Fatal the whole WASM process.

Use an atomic counter to give each listener a distinct name
(Tailscale-IPN-1, Tailscale-IPN-2, …). The connect() path is
unchanged: in the wasm/tsconnect build all LocalAPI calls go through
the in-process httptest handler, so connect() is never called.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

safesocket/safesocket_js.go

@@ -5,15 +5,22 @@ package safesocket
 
 import (
 	"context"
+	"fmt"
 	"net"
+	"sync/atomic"
 
 	"github.com/akutz/memconn"
 )
 
 const memName = "Tailscale-IPN"
 
+// memSeq ensures each IPN instance in the same WASM process gets a distinct
+// memconn address, so concurrent instances do not conflict on the registry.
+var memSeq atomic.Int64
+
 func listen(path string) (net.Listener, error) {
-	return memconn.Listen("memu", memName)
+	name := fmt.Sprintf("%s-%d", memName, memSeq.Add(1))
+	return memconn.Listen("memu", name)
 }
 
 func connect(ctx context.Context, _ string) (net.Conn, error) {