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.

scenes/level_village_1.tscn

@@ -54,7 +54,6 @@ color_ramp = SubResource("GradientTexture1D_f1fvy")
 resource_local_to_scene = true
 script = ExtResource("4_84xgs")
 LevelName = "LEVEL_1_NAME"
-ScenePath = "res://scenes/level_village_1.tscn"
 metadata/_custom_type_script = "uid://cp68km8bykymb"
 
 [sub_resource type="Resource" id="Resource_0nwt7"]