feat(ui): HUD visibility groups — gameplay vs implant layers (D-170)

New HudGroups autoload manages show/hide of related HUD elements as
groups. Exclusive groups (gameplay, implant) are mutually exclusive —
opening the star map hides stance indicator, minimap, interaction
prompts, etc. Closing it restores them.

Gameplay nodes registered in main.gd _ready(). Star map registers as
implant group and uses toggle_group() instead of direct visibility.

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

client/ui/star_map.gd

@@ -118,6 +118,9 @@ func _ready() -> void:
 	_info_panel.mouse_filter = Control.MOUSE_FILTER_IGNORE
 	add_child(_info_panel)
 
+	# D-170: Register with HUD visibility groups
+	HudGroups.register(self, "implant")
+
 	_load_data()
 	if _data_loaded:
 		_compute_layout()
@@ -140,9 +143,10 @@ func set_insert_active(active: bool) -> void:
 		visible = false
 
 
-## Toggle visibility (e.g., from a keybind or button).
+## Toggle visibility via HUD group system (D-170).
+## Shows implant group (hides gameplay) or vice versa.
 func toggle_visible() -> void:
-	visible = not visible
+	HudGroups.toggle_group("implant")
 	if visible:
 		_dirty = true