feat(ui): introduce MetaScreen pattern foundation (#618, #680)

Establishes the faux-game-menu base pattern for non-diegetic UI,
analogous to ImplantApp but for pre-gameplay and meta-overlay
screens (main menu, character creation, settings, debug console,
bug report, loading screen). Workstream 1 of the MetaScreen
refactor — foundation only, no screen migrations yet.

- client/ui/meta/meta_screen.gd: base class (Control) with
  HIDDEN/OPENING/OPEN/CLOSING phase tracking, three orthogonal
  policy booleans (pauses_sim, closable_by_escape, captures_input),
  open/close lifecycle, on_escape contract, closed + escape_pressed
  signals, subclass hooks (on_open, on_close).
- client/ui/meta/meta_stack.gd: autoload coordinator. Overlay stack
  with push/pop/top/is_active; handle_escape chain; sim-pause
  coordination via SimBridge when pauses_sim=true; meta_active_changed
  signal. All class references kept inside method bodies — no
  top-level class_name refs, matching HudGroups / GameState
  autoload parse-order discipline.
- client/scripts/character_profile.gd: Resource wrapping the
  visual descriptor with bookmark_id and start_location_id.
  Target of the creation_confirmed signal once the character
  creation flow migrates.
- client/project.godot: MetaStack registered as autoload after
  HudGroups, before ImplantRegistry.

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

client/project.godot

@@ -29,6 +29,7 @@ FogState="*res://scripts/autoloads/fog_state.gd"
 AudioManager="*res://scripts/autoloads/audio_manager.gd"
 SessionManager="*res://scripts/autoloads/session_manager.gd"
 HudGroups="*res://scripts/autoloads/hud_groups.gd"
+MetaStack="*res://ui/meta/meta_stack.gd"
 ImplantRegistry="*res://ui/implant/implant_registry.gd"
 HardwareDetector="*res://ui/hardware_detector.gd"