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/LevelCatalogue.cs

@@ -0,0 +1,54 @@
+using System.Collections.Generic;
+using System.Linq;
+using Godot;
+using Godot.Collections;
+using Mr.BrickAdventures.Progression;
+
+namespace Mr.BrickAdventures.scripts.Resources;
+
+/// <summary>
+/// The authored list of Levels in campaign order, and the bridge from a <see cref="LevelId"/> back to
+/// the Godot scene that implements it.
+/// </summary>
+/// <remarks>
+/// Not a domain port. Nothing in the progression domain resolves a Level to a scene — the domain only
+/// needs the ordering, which it takes as a <see cref="Campaign"/>. Scene lookup is wanted solely by
+/// the composition root, which is engine-side anyway.
+/// </remarks>
+[GlobalClass]
+public partial class LevelCatalogue : Resource
+{
+    /// <summary>
+    /// Levels in campaign order. A Level absent from this list is not reachable, even if its
+    /// <see cref="LevelResource"/> exists.
+    /// </summary>
+    [Export]
+    public Array<LevelResource> Levels { get; set; } = [];
+
+    /// <summary>
+    /// The ordering the domain works from. Throws if the authored data is inconsistent — a blank or
+    /// duplicated id cannot produce a <see cref="Campaign"/>, so a mistake here fails at boot rather
+    /// than corrupting a save later.
+    /// </summary>
+    public Campaign ToCampaign()
+    {
+        var ids = new List<LevelId>();
+
+        for (var i = 0; i < Levels.Count; i++)
+        {
+            var level = Levels[i];
+            if (level == null)
+                throw new System.InvalidOperationException(
+                    $"LevelCatalogue has an empty slot at index {i}.");
+
+            ids.Add(LevelId.Of(level.Id));
+        }
+
+        return Campaign.Of(ids);
+    }
+
+    public LevelResource Find(LevelId id) =>
+        Levels.FirstOrDefault(level => level != null && level.Id == id.Value);
+
+    public PackedScene SceneFor(LevelId id) => Find(id)?.Scene;
+}

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; }
+}