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

@@ -0,0 +1,93 @@
+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
+## editor-first rule does not apply (that rule governs layout scenes). It owns one
+## ContentDB + one DmService and swaps screens via instantiate → inject → add_child,
+## the injection seam change_scene_to_file cannot provide.
+##
+## §2: the flow carries only plain data; NewGame.construct re-rolls the character
+## from the creation seed itself. ⛨ saga: _show_creation and _start_campaign are
+## separate, so a fourth entry path is an added arm, not surgery on existing edges.
+
+const HttpTransport = preload("res://scripts/net/http_transport.gd")
+
+const TITLE := preload("res://scenes/title/TitleScreen.tscn")
+const CREATION := preload("res://scenes/creation/CharacterCreation.tscn")
+const SHELL := preload("res://scenes/shell/MainWindowShell.tscn")
+
+# Injection seams — set BETWEEN instantiate() and add_child(), as the shell/creation
+# do for their own deps. _ready builds the real ones when nothing was supplied.
+var world: ContentDB = null
+var service: DmService = null
+var origin: Dictionary = {}
+
+var _current: Node = null
+var _http: HTTPRequest
+
+
+func _ready() -> void:
+	if world == null:
+		world = ContentDB.new()
+		world.load_from(ContentDB.default_content_root())
+	if origin.is_empty():
+		origin = ContentDB.load_json(ContentDB.origin_path("deserter"))
+	if service == null:
+		_http = HTTPRequest.new()
+		add_child(_http)
+		service = DmService.new(HttpTransport.new(_http), FallbackLibrary.new())
+	_show_title()
+
+
+func _show(next: PackedScene, inject: Callable) -> void:
+	## The one swap primitive. inject runs after instantiate() (which does NOT run
+	## _ready) and before add_child (which does) — the injection window. The outgoing
+	## screen is queue_free'd (deferred): a signal handler must never free the node
+	## still executing up its own call stack.
+	var old := _current
+	var screen := next.instantiate()
+	inject.call(screen)
+	_current = screen
+	add_child(screen)
+	if old != null:
+		old.queue_free()
+
+
+func _show_title() -> void:
+	_show(TITLE, func(s): s.menu_activated.connect(_on_menu))
+
+
+func _show_creation() -> void:
+	_show(CREATION, func(s):
+		s.world = world
+		s.origin = origin
+		s.creation_confirmed.connect(_start_campaign))
+
+
+func _show_shell(log: CanonLog, state: GameState) -> void:
+	_show(SHELL, func(s):
+		s.service = service
+		s.injected_log = log
+		s.injected_state = state)
+
+
+func _on_menu(id: StringName) -> void:
+	match id:
+		&"new_game":
+			_show_creation()
+		&"quit":
+			get_tree().quit()
+		_:
+			pass  # continue / load land in later milestones (M9); inert for now
+
+
+func _start_campaign(creation: Dictionary) -> void:
+	## Takes a plain Dictionary — does NOT assume it came from the creation screen, so
+	## a saga can synthesize one and call this directly (skipping creation).
+	var result := NewGame.construct(origin, world, creation)
+	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("Main: construct rejected a creation: %s" % str(result.get("errors", [])))
+		return
+	_show_shell(result["log"], result["state"])