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/ui/star_map.gd

@@ -118,8 +118,9 @@ func _ready() -> void:
 	_info_panel.mouse_filter = Control.MOUSE_FILTER_IGNORE
 	add_child(_info_panel)
 
-	# D-170: Register with HUD visibility groups
+	# D-170: Register with HUD layer groups
 	HudGroups.register(self, "implant/map/starchart")
+	HudGroups.app_changed.connect(_on_app_changed)
 
 	_load_data()
 	if _data_loaded:
@@ -135,20 +136,27 @@ func _process(_delta: float) -> void:
 
 
 ## Called from main.gd when insert state changes.
-## Only force-hides when insert is inactive. Does NOT auto-show — star map is
-## modal (player opens via toggle_visible()), not always-on like the minimap.
+## Called from main.gd when insert state changes.
 func set_insert_active(active: bool) -> void:
 	_insert_active = active
-	if not active:
-		visible = false
+	if not active and HudGroups.is_app_active("implant/map/starchart"):
+		HudGroups.close_app()
 
 
-## Toggle visibility via HUD group system (D-170).
-## Opens implant/map (hides gameplay) or closes it (returns to gameplay).
+## Toggle via HUD layer system (D-170).
 func toggle_visible() -> void:
 	HudGroups.toggle_app("implant/map/starchart")
-	if visible:
+
+
+## Respond to app layer changes (D-170).
+func _on_app_changed(app_path: String, mode: HudGroups.Mode) -> void:
+	if app_path != "implant/map/starchart":
+		return
+	if mode == HudGroups.Mode.FULLSCREEN or mode == HudGroups.Mode.INSERT:
+		visible = true
 		_dirty = true
+	else:
+		visible = false
 
 
 ## Return the currently selected system data, or empty dict.