feat(multiplayer): add Steam transport foundation

Game/scripts/steam_bootstrap.gd

@@ -0,0 +1,42 @@
+class_name SteamBootstrap
+extends RefCounted
+
+# Spacewar is Valve's development App ID. It is intentionally a development
+# default, never a public-server identity or discovery configuration.
+const SPACEWAR_APP_ID := 480
+const APP_ID_ENV := "COSMIC_CLASH_STEAM_APP_ID"
+
+
+static func app_id() -> int:
+	var configured := OS.get_environment(APP_ID_ENV).strip_edges()
+	if configured.is_valid_int() and int(configured) > 0:
+		return int(configured)
+	return SPACEWAR_APP_ID
+
+
+static func is_runtime_available() -> bool:
+	return OS.has_feature("steam") and ClassDB.class_exists("SteamMultiplayerPeer") and Engine.has_singleton("Steam")
+
+
+static func unavailable_reason() -> String:
+	if not OS.has_feature("steam"):
+		return "this export was not built with the steam feature"
+	if not ClassDB.class_exists("SteamMultiplayerPeer"):
+		return "SteamMultiplayerPeer is missing from this custom Godot build"
+	if not Engine.has_singleton("Steam"):
+		return "the GodotSteam Steam singleton is missing from this custom Godot build"
+	return "Steam is unavailable"
+
+
+static func initialize() -> Dictionary:
+	if not is_runtime_available():
+		return {"error": ERR_UNAVAILABLE, "reason": unavailable_reason()}
+	var steam := Engine.get_singleton("Steam")
+	# `steamInit` is deliberately called dynamically: stock Godot must be able
+	# to parse and run this project without GodotSteam symbols installed.
+	var result = steam.call("steamInit")
+	if result is bool and result:
+		return {"error": OK, "app_id": app_id()}
+	if result is Dictionary and bool(result.get("status", false)):
+		return {"error": OK, "app_id": app_id()}
+	return {"error": ERR_CANT_CONNECT, "reason": "Steam initialization failed for App ID %d" % app_id()}