refactor(m4c): rename GameFlow coordinator to Main, move to scenes/Main.tscn

The entry-point coordinator read as an internal name ("GameFlow" buried in
scenes/flow/), so it wasn't obvious THIS is the scene that boots first — the
title screen looked like the entry point. Renamed to the Godot-conventional
Main (scenes/Main.tscn + scripts/main.gd) so the boot scene is self-evident.
TitleScreen is unchanged and still shown BY Main as its first screen.

Also drops the hand-written gd_scene uid (now matches sibling scenes) —
closes the Minor review finding. Full suite 327/327 green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

client/tests/unit/test_game_flow.gd.uid

@@ -1 +0,0 @@
-uid://buyyn7tbjctb

client/tests/unit/test_main.gd

@@ -1,6 +1,6 @@
 extends "res://addons/gut/test.gd"
 
-const SCENE := "res://scenes/flow/GameFlow.tscn"
+const SCENE := "res://scenes/Main.tscn"
 const ContentDB = preload("res://scripts/content/content_db.gd")
 const FakeTransport = preload("res://tests/doubles/fake_transport.gd")
 const DmResponse = preload("res://scripts/net/dm_response.gd")
@@ -31,7 +31,7 @@ func _creation() -> Dictionary:
 
 func test_boots_into_the_title():
 	var flow := _flow()
-	assert_true(flow._current is TitleScreen, "GameFlow opens on the title")
+	assert_true(flow._current is TitleScreen, "Main opens on the title")
 
 
 func test_new_game_shows_creation():
@@ -53,7 +53,7 @@ func _assert_stays_on_creation(bad: Dictionary, why: String) -> void:
 	var flow := _flow()
 	flow._current.emit_signal("menu_activated", &"new_game")
 	# Drive the REAL signal with an invalid dict. The CTA gates this out via the UI,
-	# but §2 forbids GameFlow trusting the caller and §13 forbids surfacing an error.
+	# but §2 forbids Main trusting the caller and §13 forbids surfacing an error.
 	flow._current.emit_signal("creation_confirmed", bad)
 	assert_false(flow._current is MainWindowShell, "a failed construct never reaches the shell (§13): %s" % why)
 	assert_true(flow._current is CharacterCreation, "the player stays on the valid creation screen: %s" % why)

client/tests/unit/test_main.gd.uid

@@ -0,0 +1 @@
+uid://17abdh3almry