tsconnect: harden JS bridge callbacks

Contain synchronous and asynchronous JS failures, make Drive cancellation fence response callbacks, and keep netmap snapshots current across peer changes and watcher restarts.

Co-Authored-By: gpt-5.6-sol <noreply@openai.com>

cmd/tsconnect/wasmbuild/printtags/main.go

@@ -0,0 +1,14 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+package main
+
+import (
+	"fmt"
+
+	"tailscale.com/cmd/tsconnect/wasmbuild"
+)
+
+func main() {
+	fmt.Print(wasmbuild.Tags())
+}