refactor(ui): migrate 6 meta screens to MetaScreen pattern (Workstream 2)

Relocates main_menu, character_creation, settings_dialog, debug_console,
bug_report_dialog, loading_screen from flat client/ui/ into structured
client/ui/meta/screens/<name>/. All six now extend MetaScreen instead
of Control; the base handles open/close lifecycle, visibility,
captures_input, and — for overlays — the sim-pause contract.

Screen policies set per Tyre's proposal:
- settings_dialog: pauses_sim=false, PUSHES onto MetaStack
- debug_console: pauses_sim=true, PUSHES (D-088 routing via base)
- bug_report_dialog: pauses_sim=true, PUSHES
- loading_screen: closable_by_escape=false, PUSHES
- main_menu, character_creation: scene-roots, extend MetaScreen for
  the lifecycle contract only, do NOT push onto the stack

character_creation stays at its current surface (tabs, descriptor,
creation_confirmed signal unchanged). Tab consolidation and
CharacterProfile migration happen in Workstreams 5 and 6.

Knock-on changes:
- main.tscn ModalLayer CanvasLayer renamed to MetaLayer; main.gd
  @onready refs updated; constants.gd comment updated; test_client_p3
  and test_ui_framework_sprint15 assertions updated; test_monologue_display
  and .tscn header comments updated.
- OPEN_MENU handler now pushes settings_dialog onto MetaStack before
  calling open(). Full ESC priority chain lands in Workstream 4.
- atlas_app.gd: _unhandled_key_input signature widened from
  InputEventKey to InputEvent with an is-check, per Godot 4 API. Pre-
  existing narrowing was silently tolerated until main.tscn started
  fully instantiating under the new pattern.
- test_client_p3: entity_renderer type annotations corrected from
  ColorRect to Sprite2D (stale since a prior refactor); facing
  indicator rotation assertion switched to angle_difference() for
  modular-safe comparison.

Verification:
- gdlint client/scripts/ client/ui/ — zero problems
- godot --headless --path client --quit — no SCRIPT ERROR
- test_client_p3: 24/24 pass
- test_ui_framework_sprint15: 54/54 pass
- test_implant_nav_stack: 52/52 pass
- test_implant_registry: 42/42 pass
- test_implant_app_lifecycle: 36/36 pass

Workstream 1 foundation (84105916) remains unchanged. Workstreams 3-8
follow: protocol layer, Option A sequencing, 3-tab restructure,
Bookmark tab, location picker, Skills stub.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/scripts/constants.gd

@@ -40,7 +40,7 @@ const CANVAS_UI: int = 20  # CanvasLayer number for UILayer
 #
 # MODAL SCOPE (CanvasLayer 30)
 # Full-screen overlays: pause, inventory modal, death screen.
-const CANVAS_MODAL: int = 30  # CanvasLayer number for ModalLayer
+const CANVAS_MODAL: int = 30  # CanvasLayer number for MetaLayer
 #
 # Rendering ceiling: 10 floors (25m) above current floor.
 # Above this: no sprites, ground shadows + environmental effects only.

client/scripts/main.gd

@@ -30,10 +30,10 @@ var _dialogue: DialogueCoordinator  # #775: dialogue consumers + signal handlers
 @onready var examine_display = $InsertOverlay/ExamineDisplay  # #174: examine result overlay
 @onready var journal_panel = $InsertOverlay/JournalPanel  # #264: knowledge journal (D-041)
 @onready var debug_overlay = $UILayer/DebugOverlay  # #511: F3 debug overlay
-@onready var bug_report_dialog = $ModalLayer/BugReportDialog  # #495: F12 WRONG button
-@onready var settings_dialog = $ModalLayer/SettingsDialog  # #528: audio settings (ESC/OPEN_MENU)
-@onready var loading_screen = $ModalLayer/LoadingScreen  # #257: blocking overlay during load
-@onready var debug_console = $ModalLayer/DebugConsole  # #581: tilde debug console
+@onready var bug_report_dialog = $MetaLayer/BugReportDialog  # #495: F12 WRONG button
+@onready var settings_dialog = $MetaLayer/SettingsDialog  # #528: audio settings (ESC/OPEN_MENU)
+@onready var loading_screen = $MetaLayer/LoadingScreen  # #257: blocking overlay during load
+@onready var debug_console = $MetaLayer/DebugConsole  # #581: tilde debug console
 @onready var news_ticker = $UILayer/NewsTicker  # #592: scrolling headline bar (D-049 z-7)
 
 
@@ -271,6 +271,7 @@ func _process(delta: float) -> void:
 				if settings_dialog.is_open():
 					settings_dialog.close()
 				else:
+					MetaStack.push(settings_dialog)
 					settings_dialog.open()
 			continue
 		if input.action == InputMapper.Action.INTERACT: