cmd/tailscaled,ipn/{auditlog,desktop,ipnext,ipnlocal},tsd: extract LocalBackend extension interfaces and implementation

In this PR, we refactor the LocalBackend extension system, moving from direct callbacks to a more organized extension host model.

Specifically, we:
- Extract interface and callback types used by packages extending LocalBackend functionality into a new ipn/ipnext package.
- Define ipnext.Host as a new interface that bridges extensions with LocalBackend.
  It enables extensions to register callbacks and interact with LocalBackend in a concurrency-safe, well-defined, and controlled way.
- Move existing callback registration and invocation code from ipnlocal.LocalBackend into a new type called ipnlocal.ExtensionHost,
  implementing ipnext.Host.
- Improve docs for existing types and methods while adding docs for the new interfaces.
- Add test coverage for both the extracted and the new code.
- Remove ipn/desktop.SessionManager from tsd.System since ipn/desktop is now self-contained.
- Update existing extensions (e.g., ipn/auditlog and ipn/desktop) to use the new interfaces where appropriate.

We're not introducing new callback and hook types (e.g., for ipn.Prefs changes) just yet, nor are we enhancing current callbacks,
such as by improving conflict resolution when more than one extension tries to influence profile selection via a background profile resolver.
These further improvements will be submitted separately.

Updates #12614
Updates tailscale/corp#27645
Updates tailscale/corp#26435
Updates tailscale/corp#18342

Signed-off-by: Nick Khyl <nickk@tailscale.com>

cmd/k8s-operator/depaware.txt

@@ -815,8 +815,8 @@ tailscale.com/cmd/k8s-operator dependencies: (generated by github.com/tailscale/
         tailscale.com/internal/noiseconn                             from tailscale.com/control/controlclient
         tailscale.com/ipn                                            from tailscale.com/client/local+
         tailscale.com/ipn/conffile                                   from tailscale.com/ipn/ipnlocal+
-     💣 tailscale.com/ipn/desktop                                    from tailscale.com/ipn/ipnlocal+
      💣 tailscale.com/ipn/ipnauth                                    from tailscale.com/ipn/ipnlocal+
+        tailscale.com/ipn/ipnext                                     from tailscale.com/ipn/ipnlocal
         tailscale.com/ipn/ipnlocal                                   from tailscale.com/ipn/localapi+
         tailscale.com/ipn/ipnstate                                   from tailscale.com/client/local+
         tailscale.com/ipn/localapi                                   from tailscale.com/tsnet+

cmd/tailscaled/depaware.txt

@@ -273,8 +273,9 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/ipn                                            from tailscale.com/client/local+
    W    tailscale.com/ipn/auditlog                                   from tailscale.com/cmd/tailscaled
         tailscale.com/ipn/conffile                                   from tailscale.com/cmd/tailscaled+
-     💣 tailscale.com/ipn/desktop                                    from tailscale.com/cmd/tailscaled+
+   W 💣 tailscale.com/ipn/desktop                                    from tailscale.com/cmd/tailscaled
      💣 tailscale.com/ipn/ipnauth                                    from tailscale.com/ipn/ipnlocal+
+        tailscale.com/ipn/ipnext                                     from tailscale.com/ipn/auditlog+
         tailscale.com/ipn/ipnlocal                                   from tailscale.com/cmd/tailscaled+
         tailscale.com/ipn/ipnserver                                  from tailscale.com/cmd/tailscaled
         tailscale.com/ipn/ipnstate                                   from tailscale.com/client/local+

cmd/tailscaled/tailscaled_windows.go

@@ -45,7 +45,7 @@ import (
 	"tailscale.com/drive/driveimpl"
 	"tailscale.com/envknob"
 	_ "tailscale.com/ipn/auditlog"
-	"tailscale.com/ipn/desktop"
+	_ "tailscale.com/ipn/desktop"
 	"tailscale.com/logpolicy"
 	"tailscale.com/logtail/backoff"
 	"tailscale.com/net/dns"
@@ -337,13 +337,6 @@ func beWindowsSubprocess() bool {
 
 	sys.Set(driveimpl.NewFileSystemForRemote(log.Printf))
 
-	if sessionManager, err := desktop.NewSessionManager(log.Printf); err == nil {
-		sys.Set(sessionManager)
-	} else {
-		// Errors creating the session manager are unexpected, but not fatal.
-		log.Printf("[unexpected]: error creating a desktop session manager: %v", err)
-	}
-
 	publicLogID, _ := logid.ParsePublicID(logID)
 	err = startIPNServer(ctx, log.Printf, publicLogID, sys)
 	if err != nil {