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.

tests/_setup_editor_preview.tscn

@@ -0,0 +1,6 @@
+[gd_scene load_steps=2 format=3]
+
+[ext_resource type="Script" path="res://tests/_setup_editor_preview.gd" id="1"]
+
+[node name="SetupEditorPreview" type="Node2D"]
+script = ExtResource("1")