feat(multiplayer): Phase 1 transport, connection, and lobby

Lands tasks 1.0-1.8 of multiplayer-todo.md: the pure-function test runner,
net_codec (wire format quantizers/pack-unpack), NetworkManager (ENet
transport, manual polling, min-RTT clock sync), MatchNet (handshake,
protocol/tick-rate gating, roster with team+ready state), lobby.tscn (team
columns, switch team, ready toggle), server_boot.tscn (headless dedicated
server with structured logging and an overrun watchdog), and main_menu.gd's
Host/Join-by-IP UI (connecting overlay, cancel, bounded failure path).

Followed by an adversarial review (Opus subagent) that found and fixed two
real bugs - an unvalidated player_name broadcast that let one client's
oversized name head-of-line-block the reliable channel for everyone, and a
server-side roster leak across a host/re-host cycle - plus three gaps in
the test suite itself where a claim of "verified" wasn't actually backed
by what the test checked. All five two-process smoke tests plus the
pure-function suite are green with the strengthened assertions in place.

Game/tests/lobby_smoke.gd

@@ -0,0 +1,79 @@
+extends Node
+
+# Manual two-process smoke test for lobby.tscn (task 1.5). BOTH roles load
+# lobby.tscn as their actual current_scene via change_scene_to_file —
+# matching how main_menu.gd's Host/Join flow (task 1.7) really gets a
+# player there — rather than instantiating it as a child of this driver.
+# That distinction matters: change_scene_to_file() operates on
+# get_tree().current_scene, and calling it from a node that ISN'T an
+# ancestor-chain match for current_scene (as an earlier draft of this test
+# did, by add_child()-ing lobby.tscn under this driver) hung completely
+# on disconnect — see multiplayer-todo.md §9 gotcha 27.
+#
+# The host role loading lobby.tscn is deliberate, not an oversight: a
+# *dedicated* server (server_boot.tscn) never loads it, but a self-hosting
+# player clicking main_menu.gd's Host button does — NetworkManager.host()
+# then _leave_to_lobby(), landing them on lobby.gd's is_server branch (a
+# read-only view of the roster, no team/ready controls). An earlier
+# version of this test skipped that branch entirely on the mistaken
+# assumption that "host" here meant "dedicated server"; adversarial review
+# caught that it left a real, production-reachable code path untested.
+#
+# Not part of tests/test_runner.tscn — needs real ENet peers. Run:
+#
+#   godot --headless --path Game res://tests/lobby_smoke.tscn -- --role=host
+#   godot --headless --path Game res://tests/lobby_smoke.tscn -- --role=client
+
+const PORT := 7806
+
+var _role := ""
+
+
+func _ready() -> void:
+	for arg in OS.get_cmdline_user_args():
+		if arg.begins_with("--role="):
+			_role = arg.substr("--role=".length())
+
+	match _role:
+		"host":
+			var err := NetworkManager.host(PORT)
+			if err != OK:
+				print("SMOKE FAIL: host() failed: %s" % error_string(err))
+				get_tree().quit(1)
+				return
+			print("SMOKE: hosting on port %d" % PORT)
+			get_tree().change_scene_to_file.call_deferred("res://scenes/lobby.tscn")
+			var host_hooks := preload("res://tests/lobby_test_hooks.gd").new()
+			get_tree().root.add_child.call_deferred(host_hooks)
+			host_hooks.run_host_test.call_deferred()
+		"client":
+			MatchNet.local_player_name = "Carol"
+			var err := NetworkManager.join("127.0.0.1", PORT)
+			if err != OK:
+				print("SMOKE FAIL: join() failed: %s" % error_string(err))
+				get_tree().quit(1)
+				return
+			# Real usage (task 1.7): main_menu.gd will call this same
+			# change_scene_to_file after NetworkManager.join() succeeds — but
+			# not from this test's own _ready(), which the tree is still in
+			# the middle of processing (Godot rejects a synchronous
+			# change_scene_to_file mid node-add with "Parent node is busy").
+			# call_deferred sidesteps that; main_menu.gd's real button-press
+			# handler won't have this problem since it isn't called from
+			# inside _ready().
+			get_tree().change_scene_to_file.call_deferred("res://scenes/lobby.tscn")
+			var hooks := preload("res://tests/lobby_test_hooks.gd").new()
+			get_tree().root.add_child.call_deferred(hooks)   # sibling of current_scene, not a child of it -- survives the swap above
+			hooks.run_client_test.call_deferred()
+		_:
+			print("SMOKE FAIL: missing or unrecognised --role=")
+			get_tree().quit(1)
+			return
+
+
+func _process(_delta: float) -> void:
+	NetworkManager.poll()
+
+
+func _physics_process(_delta: float) -> void:
+	NetworkManager.poll()