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/scenes/character_creation.tscn

@@ -1,6 +1,6 @@
 [gd_scene load_steps=3 format=3 uid="uid://char_creation_scene_sr"]
 
-[ext_resource type="Script" path="res://ui/character_creation.gd" id="1_charcreation"]
+[ext_resource type="Script" path="res://ui/meta/screens/character_creation/character_creation.gd" id="1_charcreation"]
 
 ; #705: Character creation screen — 3D preview + 5-tab customisation panel.
 ; SubViewport renders CharacterVisual live. Tab panel: Body/Head/Hair/Clothing/Accessories.

client/scenes/main.tscn

@@ -187,17 +187,17 @@ script = ExtResource("10_cursor")
 
 ; --- Modal layer (CanvasLayer 30) ---
 ; Full-screen overlays: pause menu, inventory modal, death screen.
-[node name="ModalLayer" type="CanvasLayer" parent="."]
+[node name="MetaLayer" type="CanvasLayer" parent="."]
 layer = 30
 
 ; #495: WRONG button (F12) — bug report capture dialog
-[node name="BugReportDialog" parent="ModalLayer" instance=ExtResource("19_bugreport")]
+[node name="BugReportDialog" parent="MetaLayer" instance=ExtResource("19_bugreport")]
 
 ; #528: Audio settings dialog — 5-bus volume sliders, ESC/OPEN_MENU to toggle
-[node name="SettingsDialog" parent="ModalLayer" instance=ExtResource("21_settings")]
+[node name="SettingsDialog" parent="MetaLayer" instance=ExtResource("21_settings")]
 
 ; #257: Loading screen — full-screen overlay during save/load round-trip
-[node name="LoadingScreen" parent="ModalLayer" instance=ExtResource("26_loading")]
+[node name="LoadingScreen" parent="MetaLayer" instance=ExtResource("26_loading")]
 
 ; #581: Debug console — tilde key toggles, bottom 40% of screen
-[node name="DebugConsole" parent="ModalLayer" instance=ExtResource("27_debug_console")]
+[node name="DebugConsole" parent="MetaLayer" instance=ExtResource("27_debug_console")]

client/scenes/main_menu.tscn

@@ -1,6 +1,6 @@
 [gd_scene load_steps=2 format=3 uid="uid://main_menu_sr"]
 
-[ext_resource type="Script" path="res://ui/main_menu.gd" id="1_mainmenu"]
+[ext_resource type="Script" path="res://ui/meta/screens/main_menu/main_menu.gd" id="1_mainmenu"]
 
 ; Main menu — New Game / Continue / Quit.
 ; #258: D-085 per-game save directory created on New Game.