feat(ui): implant app hierarchy in HudGroups (D-170)

Implant apps use hierarchical groups: implant/map, implant/wiki,
implant/journal, etc. Apps are mutually exclusive with each other
and with gameplay. Opening implant/map hides gameplay + any other
implant app. Closing returns to gameplay.

API: open_app(), close_app(), toggle_app(), is_app_active().
Star map uses implant/map. Future apps register their own group.

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

client/ui/star_map.gd

@@ -119,7 +119,7 @@ func _ready() -> void:
 	add_child(_info_panel)
 
 	# D-170: Register with HUD visibility groups
-	HudGroups.register(self, "implant")
+	HudGroups.register(self, "implant/map")
 
 	_load_data()
 	if _data_loaded:
@@ -144,9 +144,9 @@ func set_insert_active(active: bool) -> void:
 
 
 ## Toggle visibility via HUD group system (D-170).
-## Shows implant group (hides gameplay) or vice versa.
+## Opens implant/map (hides gameplay) or closes it (returns to gameplay).
 func toggle_visible() -> void:
-	HudGroups.toggle_group("implant")
+	HudGroups.toggle_app("implant/map")
 	if visible:
 		_dirty = true