Add planet setups as authored resources (#42)

PlanetSetup/PlanetSetupSpawn define a curated capture-point/enemy
layout for a surface profile; PlanetSetups scans res://PlanetSetups/
at first use (not a hardcoded preload table) so dropping in a new
.tres is the only step needed to add one, and resolves the same
setup for the same planet deterministically, same pattern as
WorldGen.planet_data_at_chunk. PlanetSurfaceController composes a
PlanetSetupHost onto the landed surface alongside the existing
resource-provider feature scene.

Ships two starter setups (rocky_default, gas_default) with
placeholder capture-point markers; #43 replaces the markers with the
real CapturePoint scene through the same apply_setup() entry point.

Part of the #40 conquest-mechanic breakdown.

PlanetSetups/rocky_default.tres

@@ -0,0 +1,22 @@
+[gd_resource type="Resource" script_class="PlanetSetup" load_steps=4 format=3]
+
+[ext_resource type="Script" path="res://PlanetSetup.gd" id="1"]
+[ext_resource type="Script" path="res://PlanetSetupSpawn.gd" id="2"]
+
+[sub_resource type="Resource" id="Resource_r1"]
+script = ExtResource("2")
+enemy_id = "chaser"
+position = Vector2(450, 250)
+
+[sub_resource type="Resource" id="Resource_r2"]
+script = ExtResource("2")
+enemy_id = "chaser"
+position = Vector2(-600, -350)
+
+[resource]
+script = ExtResource("1")
+setup_id = "rocky_default"
+display_name = "Rocky Default"
+surface_profile_id = "rocky"
+capture_points = PackedVector2Array(700, 100, -650, 300, 50, -750)
+enemy_spawns = Array[Resource]([SubResource("Resource_r1"), SubResource("Resource_r2")])