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/test_runner.gd

@@ -0,0 +1,81 @@
+extends Node
+
+# Headless test runner (task 1.0). Discovers every *.gd under tests/cases/,
+# instances it, and calls every test_*() method by name — a test case is
+# picked up by dropping a file in that folder, not by registering it here.
+# Run with: godot --headless --path Game res://tests/test_runner.tscn
+
+const CASES_DIR := "res://tests/cases"
+const TestCase = preload("res://tests/test_case.gd")
+
+func _ready() -> void:
+	var case_paths := _discover_case_paths()
+	var total := 0
+	var failed := 0
+	var failure_messages: Array[String] = []
+
+	for path in case_paths:
+		# Adversarial-review regression: a case file with a parse/compile
+		# error used to hang the whole runner forever. load() on a broken
+		# script does NOT return null here — it returns a non-null but
+		# uninstantiable GDScript resource, so a plain null check doesn't
+		# catch it; calling .new() on it throws "Invalid call: Nonexistent
+		# function 'new'", severe enough to abort _ready() entirely without
+		# ever reaching quit(). can_instantiate() is the real guard.
+		var script: GDScript = load(path)
+		if script == null or not script.can_instantiate():
+			failed += 1
+			failure_messages.append("%s: failed to load (parse/compile error — see SCRIPT ERROR above)" % path.get_file())
+			continue
+		var instance = script.new()
+		if instance == null:
+			failed += 1
+			failure_messages.append("%s: script.new() returned null" % path.get_file())
+			continue
+
+		for method in instance.get_method_list():
+			var method_name: String = method["name"]
+			if not method_name.begins_with("test_"):
+				continue
+			total += 1
+			instance.failures.clear()
+			instance.assertions_made = 0
+			instance.call(method_name)
+			# Adversarial-review regression: GDScript has no exceptions, so
+			# a runtime error partway through a test (before it reaches its
+			# first assert_*) just logs a SCRIPT ERROR and returns —
+			# `failures` stays empty exactly as if every assertion passed,
+			# and this used to count as a PASS. A test that completes
+			# having made zero assertions is itself a failure: it proved
+			# nothing, whether because it crashed early or was just never
+			# written to assert anything.
+			if instance.assertions_made == 0:
+				failed += 1
+				failure_messages.append("%s.%s: made no assertions (crashed before the first assert_*, or the test itself is incomplete)" % [path.get_file(), method_name])
+			elif not instance.failures.is_empty():
+				failed += 1
+				for f in instance.failures:
+					failure_messages.append("%s.%s: %s" % [path.get_file(), method_name, f])
+
+	print("Ran %d tests from %d case file(s), %d failed" % [total, case_paths.size(), failed])
+	for message in failure_messages:
+		print("  FAIL: " + message)
+
+	get_tree().quit(1 if failed > 0 else 0)
+
+
+func _discover_case_paths() -> Array[String]:
+	var paths: Array[String] = []
+	var dir := DirAccess.open(CASES_DIR)
+	if dir == null:
+		push_error("Cannot open " + CASES_DIR)
+		return paths
+	dir.list_dir_begin()
+	var file_name := dir.get_next()
+	while file_name != "":
+		if file_name.ends_with(".gd") and not dir.current_is_dir():
+			paths.append(CASES_DIR + "/" + file_name)
+		file_name = dir.get_next()
+	dir.list_dir_end()
+	paths.sort()
+	return paths