feat(multiplayer): add Steam transport foundation

Game/tests/cases/test_net_transport.gd

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+extends "res://tests/test_case.gd"
+
+const EnetTransport = preload("res://scripts/enet_transport.gd")
+const SteamBootstrap = preload("res://scripts/steam_bootstrap.gd")
+const SteamTransport = preload("res://scripts/steam_transport.gd")
+
+func test_enet_is_available_without_steam() -> void:
+	var transport = EnetTransport.new()
+	assert_true(transport.is_available(), "ENet remains available in a stock Godot build")
+	assert_eq(transport.transport_id(), "enet", "stable selection key")
+
+
+func test_steam_development_app_id_has_a_safe_default() -> void:
+	assert_eq(SteamBootstrap.app_id(), SteamBootstrap.SPACEWAR_APP_ID, "Spacewar is the local-development default")
+	assert_true(SteamBootstrap.app_id() > 0, "Steam bootstrap never uses an invalid app ID")
+
+
+func test_stock_build_refuses_steam_without_falling_back_to_enet() -> void:
+	var transport = SteamTransport.new()
+	if transport.is_available():
+		assert_true(true, "a custom Steam build is validated by the separate Steam export check")
+		return
+	var result: Dictionary = transport.create_server(7777, 2)
+	assert_eq(int(result.error), ERR_UNAVAILABLE, "Steam request fails explicitly when its custom build is absent")
+	assert_true(not result.has("peer") or result.peer == null, "an unavailable Steam request never returns an ENet peer")
+	assert_true(not transport.unavailable_reason().is_empty(), "failure tells an operator what is missing")

Game/tests/steam_template_smoke.gd

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+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)

Game/tests/steam_template_smoke.tscn

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+[gd_scene load_steps=2 format=3]
+
+[ext_resource type="Script" path="res://tests/steam_template_smoke.gd" id="1"]
+
+[node name="SteamTemplateSmoke" type="Node"]
+script = ExtResource("1")