feat(levels): level catalogue with stable ids (slice 7)

LevelResource gains a stable Id and swaps the string ScenePath for an
editor-tracked PackedScene. The old path field was never read and had
rotted - forest_1 pointed at level_forest_5.tscn, which doesn't exist,
and borrowed LEVEL_5_NAME so it would have displayed "VILLAGE - 5".

LevelCatalogue lists levels in campaign order and produces the domain's
Campaign; an authoring mistake fails at boot because Campaign.Of rejects
blank and duplicate ids. Not a domain port - nothing in the domain
resolves a level to a scene.

All 8 scenes now have a resource; campaign keeps the existing 6
(tutorial_1, village_1..5). village_6 and forest_1 are catalogued but
deliberately out of the campaign.

GameManager.LevelScenes stays until slice 10 rewrites GameManager.

scripts/Resources/LevelResource.cs

@@ -2,12 +2,33 @@ using Godot;
 
 namespace Mr.BrickAdventures.scripts.Resources;
 
+/// <summary>
+/// One authored Level: its stable id, its display name key, and the scene to load.
+/// </summary>
+/// <remarks>
+/// <para>
+/// <see cref="Id"/> is what Playthroughs and Best Times reference, so it must never change once a
+/// build has shipped — unlike position in a list, which is what the old
+/// <c>GameManager.LevelScenes</c> array used and which every save depended on.
+/// </para>
+/// <para>
+/// The scene is a <see cref="PackedScene"/> rather than a path string. The previous
+/// <c>ScenePath</c> field was never read by anything and had rotted: <c>forest_1.tres</c> pointed at
+/// <c>res://scenes/level_forest_5.tscn</c>, which does not exist. An exported reference is tracked by
+/// the editor and cannot drift.
+/// </para>
+/// </remarks>
 [GlobalClass]
 public partial class LevelResource : Resource
 {
-    [Export] 
+    /// <summary>Stable identity. Referenced by saves — do not change after shipping.</summary>
+    [Export]
+    public string Id { get; set; } = string.Empty;
+
+    /// <summary>Translation key for the player-facing name. Labels auto-translate it.</summary>
+    [Export]
     public string LevelName { get; set; } = string.Empty;
 
-    [Export] 
-    public string ScenePath { get; set; } = string.Empty;
-}
+    [Export]
+    public PackedScene Scene { get; set; }
+}