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/scripts/net_debug_overlay.gd

@@ -0,0 +1,43 @@
+extends CanvasLayer
+
+# Autoload: toggleable network debug overlay (F4 by default — see
+# toggle_net_overlay in project.godot's [input]). Read-only against
+# NetworkManager's clock state (task 1.8). Mirrors perf_overlay.gd's pattern
+# — headless-guarded, hidden by default, no gameplay-state writes.
+
+var _label: Label
+
+
+func _ready() -> void:
+	if DisplayServer.get_name() == "headless":
+		set_process(false)
+		return
+	layer = 100
+	_label = Label.new()
+	_label.add_theme_font_size_override("font_size", 14)
+	_label.add_theme_color_override("font_color", Color(0.5, 0.8, 1.0))
+	_label.add_theme_color_override("font_shadow_color", Color(0, 0, 0, 0.85))
+	_label.add_theme_constant_override("shadow_offset_x", 1)
+	_label.add_theme_constant_override("shadow_offset_y", 1)
+	_label.position = Vector2(12, 90)
+	_label.visible = false
+	add_child(_label)
+
+
+func _unhandled_input(event: InputEvent) -> void:
+	if event.is_action_pressed("toggle_net_overlay") and _label:
+		_label.visible = not _label.visible
+
+
+func _process(_delta: float) -> void:
+	if not _label or not _label.visible:
+		return
+	if NetworkManager.is_server:
+		_label.text = "NET: server, %d peer(s)" % (MatchNet.roster.size())
+	elif NetworkManager.is_client:
+		if NetworkManager.rtt_ms < 0.0:
+			_label.text = "NET: client, connecting (no clock sample yet)"
+		else:
+			_label.text = "NET: client  RTT %.1fms  clock offset %.1fms" % [NetworkManager.rtt_ms, NetworkManager.clock_offset_ms]
+	else:
+		_label.text = "NET: offline"