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/project.godot

@@ -44,6 +44,9 @@ GameSettings="*res://scripts/game_settings.gd"
 VideoSettings="*res://scripts/video_settings.gd"
 BackgroundFPS="*res://scripts/background_fps.gd"
 PerfOverlay="*res://scripts/perf_overlay.gd"
+NetworkManager="*res://scripts/network_manager.gd"
+MatchNet="*res://scripts/match_net.gd"
+NetDebugOverlay="*res://scripts/net_debug_overlay.gd"
 
 [display]
 
@@ -157,6 +160,11 @@ toggle_perf_overlay={
 "events": [Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":-1,"window_id":0,"alt_pressed":false,"shift_pressed":false,"ctrl_pressed":false,"meta_pressed":false,"pressed":false,"keycode":0,"physical_keycode":4194334,"key_label":0,"unicode":0,"location":0,"echo":false,"script":null)
 ]
 }
+toggle_net_overlay={
+"deadzone": 0.5,
+"events": [Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":-1,"window_id":0,"alt_pressed":false,"shift_pressed":false,"ctrl_pressed":false,"meta_pressed":false,"pressed":false,"keycode":0,"physical_keycode":4194335,"key_label":0,"unicode":0,"location":0,"echo":false,"script":null)
+]
+}
 
 [layer_names]