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_body_state.gd

@@ -0,0 +1,23 @@
+extends RefCounted
+
+# Plain data holder for one body's snapshot state (§2.4 of multiplayer-todo.md).
+# Deliberately not Ship/Ball themselves, and deliberately not a scene-tree
+# node — NetCodec's pack/unpack must stay callable from pure-function tests
+# with no live scene. Phase 2's snapshot writer fills one of these per body
+# per tick from the real RigidBody3D state; Phase 2's interpolator does the
+# reverse.
+#
+# avel_range must match what the sender quantised with (SHIP_AVEL_RANGE vs
+# BALL_AVEL_RANGE in net_codec.gd) — it is not carried on the wire, because
+# slot order already tells both peers which body is which (§1.3: "entities
+# are addressed by integer slot, never by path").
+
+var position := Vector3.ZERO
+var rotation := Quaternion.IDENTITY
+var linear_velocity := Vector3.ZERO
+var angular_velocity := Vector3.ZERO
+var frozen := false
+var turbo := false
+var thrust_z := 0.0          # -1..1; re-quantised to a 3-bit bin on the wire
+var stalled := false
+var avel_range := 4.0        # NetCodec.SHIP_AVEL_RANGE; set to BALL_AVEL_RANGE for the ball