Add the planet setup map editor (#48)

PlanetSetupEditor authors PlanetSetup resources: pick a surface
profile (drives the real lunar_surface background, not a blank
sheet), place/move/delete capture points and enemies from a palette
generated straight off EnemyDefinitions.all() (a new enemy definition
shows up with no editor change), pan/pinch-zoom the canvas with the
same touch idiom as GameController, then save to
res://PlanetSetups/<id>.tres via ResourceSaver - reachable now from
the main menu's Map Editor button. The mutation methods
(new_setup/load_setup/add_*/move_*/remove_*/save_setup) are the whole
model and work standalone, which is what lets
tests/test_setup_editor.gd drive placement/move/delete/save/reload
directly without simulating touch input.

Every cross-file reference is a preloaded script const rather than a
bare global class name, matching #42/#44's load-order notes - this
file is instantiated from other scripts' function bodies, never as a
script-mode test entry point itself, so it wasn't at risk the same way
the test files were, but the pattern is kept consistent throughout.

Part of the #40 conquest-mechanic breakdown. This closes out the
breakdown: #41-#48 are now all implemented, tested, and merged.

MainMenu.gd

@@ -9,10 +9,7 @@ extends Node2D
 func _ready() -> void:
 	play_button.pressed.connect(_on_play_pressed)
 	editor_button.pressed.connect(_on_editor_pressed)
-	# #48 wires the real map editor scene up to this button; until then it
-	# stays visible but explains itself instead of silently doing nothing
-	# or pointing at a scene that doesn't exist yet.
-	editor_button.enabled = false
+	editor_button.enabled = true
 	quit_button.pressed.connect(_on_quit_pressed)
 	quit_button.visible = not OS.has_feature("web")