feat(ui): PR #131 review round 4 — scene_path, schema_version, screens absorption

Three architectural review comments implemented per Tyre's proposals:

#3 — scene_path consumption (full, Option A)
- ImplantRegistry.instantiate_all(parent) loads and instantiates all
  registered apps with a declared scene_path. Manifests with empty
  scene_path are treated as metadata-only (silently skipped).
- ImplantRegistry.get_app_instance(app_path) returns the live instance.
- hud.tscn no longer direct-instances AtlasApp or EconomicsApp — an
  AppsContainer Control holds the registry-managed children.
- hud.gd._ready() calls ImplantRegistry.instantiate_all($AppsContainer).
- main.gd drops @onready vars for atlas_app/economics_app; looks up
  both from the registry at the top of _ready().

#6c — schema_version on manifest
- ImplantAppManifest: @export var schema_version: int = 1 (first field).
- ImplantRegistry: const CURRENT_SCHEMA_VERSION := 1; tiered check in
  _scan() — older-than-current emits print_verbose, newer-than-current
  emits push_warning, both proceed best-effort.
- apps/atlas/app.tres + apps/economics/app.tres: schema_version = 1.

#10 — absorb _on_screen_changed boilerplate into ImplantApp
- Base class gains _screens: Dictionary, _current_screen_id: String,
  register_screen(id, screen), current_screen_id(), and a real default
  _on_screen_changed implementation that handles leave+hide+enter+show
  with has_method guards and same-screen-replace detection.
- atlas_app: deletes _current_screen_id, _get_screen(),
  _on_screen_changed() override; on_install() collapses to construct →
  setup → wire → register_screen(id, screen) per screen. Preserves
  direct screen refs for atlas-specific signal wiring and method calls.
- economics_app: deletes same scaffolding; on_install() reduces to three
  lines (construct overview_screen, register_screen, nav.set_default).

Also: replaced Resource.get(name, default) dict-style calls with direct
property access on typed ImplantAppManifest reads (2-arg get() is
Dictionary-only; causes "Too many arguments" parse errors on Resources).
_is_valid_manifest gained a Resource type guard and a property-exists
check.

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

client/scripts/main.gd

@@ -2,6 +2,9 @@ extends Node2D
 
 const TELEPORT_DISTANCE_THRESHOLD: float = 5.0
 
+var economics_app = null  # EconomicsApp — populated in _ready() via ImplantRegistry
+var atlas_app = null  # AtlasApp — populated in _ready() via ImplantRegistry
+
 var _camera_anchored: bool = false
 var _flash_rect: ColorRect = null  # #502/#501: ephemeral screen flash overlay
 var _teleport_in_progress: bool = false  # #501/#117: forces camera snap on next frame
@@ -32,13 +35,15 @@ var _dialogue: DialogueCoordinator  # #775: dialogue consumers + signal handlers
 @onready var loading_screen = $ModalLayer/LoadingScreen  # #257: blocking overlay during load
 @onready var debug_console = $ModalLayer/DebugConsole  # #581: tilde debug console
 @onready var news_ticker = $UILayer/NewsTicker  # #592: scrolling headline bar (D-049 z-7)
-@onready var economics_app = $InsertOverlay/HUD/EconomicsApp  # #824: economics monitor (D-170)
-@onready var atlas_app = $InsertOverlay/HUD/AtlasApp  # #844: atlas implant — reach → system → planet → regional (D-191)
 
 
 func _ready() -> void:
 	print("The Settled Reach — client initialized")
 
+	# #844 D-191: Populate app refs from registry (hud._ready already called instantiate_all).
+	atlas_app = ImplantRegistry.get_app_instance("implant/map")
+	economics_app = ImplantRegistry.get_app_instance("implant/economics")
+
 	# #117: Manual lerp approach — disable Godot's built-in Camera2D smoothing.
 	camera.position_smoothing_enabled = false