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.gd

@@ -0,0 +1,69 @@
+class_name PlanetSetups
+extends RefCounted
+
+# Registry of authored PlanetSetup resources. Deliberately scans
+# res://PlanetSetups/ at first use instead of a hardcoded preload
+# dictionary (contrast PlanetDefinitions.gd), so dropping a new .tres in
+# that folder is the only step needed to add a setup - the map editor
+# (#48) writes there directly.
+
+const SETUPS_DIR: String = "res://PlanetSetups"
+# Compared against get_script() rather than an `is PlanetSetup` check - the
+# same load-order hazard as PlanetSurfaceController's setup_host: a global
+# class_name identifier can be looked up before an import pass has
+# registered it, while a direct script preload never depends on that cache.
+const PLANET_SETUP_SCRIPT: Script = preload("res://PlanetSetup.gd")
+
+static var _all: Array = []
+static var _scanned: bool = false
+
+
+static func _ensure_scanned() -> void:
+	if _scanned:
+		return
+	_scanned = true
+	var dir: DirAccess = DirAccess.open(SETUPS_DIR)
+	if dir == null:
+		return
+	dir.list_dir_begin()
+	var file_name: String = dir.get_next()
+	while file_name != "":
+		if not dir.current_is_dir() and file_name.ends_with(".tres"):
+			var res: Resource = load(SETUPS_DIR + "/" + file_name)
+			if res != null and res.get_script() == PLANET_SETUP_SCRIPT:
+				_all.append(res)
+		file_name = dir.get_next()
+	dir.list_dir_end()
+
+
+static func rescan() -> void:
+	# The map editor calls this after saving so a freshly written setup is
+	# visible without restarting the process.
+	_all = []
+	_scanned = false
+	_ensure_scanned()
+
+
+static func all() -> Array:
+	_ensure_scanned()
+	return _all
+
+
+static func setups_for_profile(profile_id: String) -> Array:
+	var result: Array = []
+	for setup in all():
+		if setup.surface_profile_id == profile_id:
+			result.append(setup)
+	return result
+
+
+static func setup_for_planet(planet_id: String, profile_id: String) -> Resource:
+	# Same pattern as WorldGen.planet_data_at_chunk: seeded purely from the
+	# planet's own stable id, so the same planet always resolves to the
+	# same setup from anywhere, with nothing stored.
+	var candidates: Array = setups_for_profile(profile_id)
+	if candidates.is_empty():
+		return null
+	var rng: RandomNumberGenerator = RandomNumberGenerator.new()
+	rng.seed = hash(planet_id)
+	return candidates[rng.randi() % candidates.size()]