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/scripts/main.gd

@@ -62,6 +62,14 @@ 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
+	for node: Control in [hud, minimap, stance_indicator, cursor_renderer,
+			interaction_prompt, interaction_list, inventory_grid, news_ticker,
+			examine_display, world_radial]:
+		if node:
+			HudGroups.register(node, "gameplay")
+
 	# #775: Initialize extracted components
 	_consumers = SnapshotConsumers.new().init({
 		"monologue_display": monologue_display,