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.

tests/test_planet_setups.gd.uid

@@ -0,0 +1 @@
+uid://d2ekw3y7ipgy2