refactor(app): harden shutdown per final review

Addresses final code review of phase-1 branch (Important I-1, I-3):

- Move plugin Enable() loop inside the same try block as GameWindow.Run,
  and wrap each Enable() in per-plugin try/catch mirroring the Disable
  loop. Previously, a plugin Enable() throwing would skip the finally
  block entirely: plugins that had already enabled would never get
  disabled, Serilog would never flush, and the exception would escape
  ungracefully. Now Enable failures are logged and contained, and
  shutdown always runs.
- Add a comment at the Get<LandBlock> call in GameWindow.OnLoad explaining
  why TryGet was avoided (the [MaybeNullWhen(false)] nullable-generic
  analysis trips TreatWarningsAsErrors).

I-2 (camera aspect doesn't update on window resize) deferred to Phase 2.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/AcDream.App/Program.cs

@@ -40,11 +40,14 @@ foreach (var result in PluginDiscovery.Scan(pluginsDir))
     Log.Information("loaded plugin {Id} ({DisplayName})", result.Manifest!.Id, result.Manifest.DisplayName);
 }
 
-foreach (var plugin in loaded)
-    plugin.Plugin!.Enable();
-
 try
 {
+    foreach (var plugin in loaded)
+    {
+        try { plugin.Plugin!.Enable(); }
+        catch (Exception ex) { Log.Error(ex, "plugin enable failed: {Id}", plugin.Manifest.Id); }
+    }
+
     using var window = new GameWindow(datDir);
     window.Run();
 }