feat(multiplayer): add Steam transport foundation

Game/tests/steam_template_smoke.gd

@@ -0,0 +1,20 @@
+extends Node
+
+# This is intentionally separate from the stock test suite: it is run only
+# by scripts/verify_steam_templates.sh, where failing to supply a custom Steam
+# executable is a setup failure rather than a regression in the ENet build.
+func _ready() -> void:
+	var failures: Array[String] = []
+	if not OS.has_feature("steam"):
+		failures.append("custom export is missing the steam feature")
+	if not ClassDB.class_exists("SteamMultiplayerPeer"):
+		failures.append("SteamMultiplayerPeer is missing")
+	if not Engine.has_singleton("Steam") and not Engine.has_singleton("SteamServer"):
+		failures.append("neither Steam nor SteamServer singleton is available")
+	if failures.is_empty():
+		print("STEAM TEMPLATE SMOKE PASS")
+		get_tree().quit(0)
+		return
+	for failure in failures:
+		printerr("STEAM TEMPLATE SMOKE FAIL: %s" % failure)
+	get_tree().quit(1)