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/scripts/main.gd

@@ -1,4 +1,4 @@
-class_name GameFlow
+class_name Main
 extends Node
 ## The single home for the game's screen sequence: Title → Creation → Shell.
 ## Script-first by design — a pure coordinator with no Control layout, so ADR 0001's
@@ -88,6 +88,6 @@ func _start_campaign(creation: Dictionary) -> void:
 	if not result.get("ok", false):
 		# Unreachable via the UI (the CTA gates on validate), but §2 forbids trusting
 		# the caller and §13 forbids surfacing an error. Log and stay put.
-		push_error("GameFlow: construct rejected a creation: %s" % str(result.get("errors", [])))
+		push_error("Main: construct rejected a creation: %s" % str(result.get("errors", [])))
 		return
 	_show_shell(result["log"], result["state"])

client/scripts/main.gd.uid

@@ -0,0 +1 @@
+uid://du7rwup648n60

client/scripts/ui/flow/game_flow.gd.uid

@@ -1 +0,0 @@
-uid://cy0r08ct0jugt