refactor(core): preserve exception in PluginDiscoveryResult + order deterministically

Addresses code quality review of 9161868:
- PluginDiscoveryResult.Error is now Exception? rather than string?,
  preserving stack traces across the plugin boundary for logging
- PluginDiscovery.Scan orders subdirectories by ordinal string comparison
  so plugin load order is reproducible across platforms

src/AcDream.Core/Plugins/PluginDiscovery.cs

@@ -3,7 +3,7 @@ namespace AcDream.Core.Plugins;
 public sealed record PluginDiscoveryResult(
     string PluginDirectory,
     PluginManifest? Manifest,
-    string? Error)
+    Exception? Error)
 {
     public bool Success => Manifest is not null && Error is null;
 }
@@ -16,7 +16,7 @@ public static class PluginDiscovery
             return Array.Empty<PluginDiscoveryResult>();
 
         var results = new List<PluginDiscoveryResult>();
-        foreach (var subdir in Directory.EnumerateDirectories(pluginsRootDirectory))
+        foreach (var subdir in Directory.EnumerateDirectories(pluginsRootDirectory).OrderBy(p => p, StringComparer.Ordinal))
         {
             var manifestPath = Path.Combine(subdir, "plugin.json");
             if (!File.Exists(manifestPath))
@@ -30,7 +30,7 @@ public static class PluginDiscovery
             }
             catch (Exception ex)
             {
-                results.Add(new PluginDiscoveryResult(subdir, null, ex.Message));
+                results.Add(new PluginDiscoveryResult(subdir, null, ex));
             }
         }
         return results;