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.

resources/levels/village/village_4.tres

@@ -1,9 +1,11 @@
-[gd_resource type="Resource" script_class="LevelResource" load_steps=2 format=3 uid="uid://cxfei28yu3buh"]
+[gd_resource type="Resource" script_class="LevelResource" load_steps=3 format=3 uid="uid://cxfei28yu3buh"]
 
-[ext_resource type="Script" uid="uid://c2h0pqhxiqswe" path="res://scripts/Resources/LevelResource.cs" id="1_h4twv"]
+[ext_resource type="Script" uid="uid://c2h0pqhxiqswe" path="res://scripts/Resources/LevelResource.cs" id="1_script"]
+[ext_resource type="PackedScene" uid="uid://bhad760x3vvco" path="res://scenes/level_village_4.tscn" id="2_scene"]
 
 [resource]
-script = ExtResource("1_h4twv")
+script = ExtResource("1_script")
+Id = "village_4"
 LevelName = "LEVEL_4_NAME"
-ScenePath = "res://scenes/level_village_4.tscn"
+Scene = ExtResource("2_scene")
 metadata/_custom_type_script = "uid://cp68km8bykymb"