refactor(ui): HudGroups z-index model + gameplay occlusion (D-170)

Replaced visibility toggling with z-index layer management. Nothing
gets hidden — fullscreen apps render on top, gameplay stays underneath.
Supports future hybrid layouts (insert-mode map alongside gameplay).

Added gameplay_occluded signal: WorldRenderer skips tile/fog/entity
updates when a fullscreen implant app covers it. Prevents wasted
render work behind opaque overlays. Other renderers can connect to
the same signal.

Modes: GAMEPLAY (z=0), INSERT (z=10), FULLSCREEN (z=20), MODAL (z=30).
Star map uses app_changed signal to toggle its own visibility based
on whether its app is active.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

client/scripts/main.gd

@@ -62,20 +62,13 @@ func _ready() -> void:
 		camera.global_position = GameState.player_position * Constants.TILE_SIZE
 		_camera_anchored = true
 
-	# D-170: Register HUD nodes into visibility groups
-	# Gameplay group — hidden when implant panels are open
+	# D-170: Register HUD nodes into layer groups
 	for node in [
-		hud,
-		minimap,
-		stance_indicator,
-		interaction_prompt,
-		interaction_list,
-		inventory_grid,
-		news_ticker,
-		examine_display,
-		world_radial
+		hud, minimap, stance_indicator, interaction_prompt,
+		interaction_list, inventory_grid, news_ticker,
+		examine_display, world_radial, cursor_renderer,
 	]:
-		if node and node is Control:
+		if node and node is CanvasItem:
 			HudGroups.register(node, "gameplay")
 
 	# #775: Initialize extracted components