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/autoloads/hud_groups.gd

@@ -0,0 +1,88 @@
+extends Node
+## HUD visibility group manager (D-170).
+##
+## Nodes register themselves into groups. When a group's visibility changes,
+## all registered nodes show/hide together. This replaces per-panel manual
+## hide/show of individual HUD elements.
+##
+## Groups:
+##   "gameplay"  — stance indicator, minimap, health/time, interaction prompts,
+##                 cursor renderer, inventory, news ticker, examine display
+##   "implant"   — star map, travel planner, GTTR reader, station profile
+##   "modal"     — settings, bug report, loading screen, debug console
+##   "debug"     — debug overlay, gauntlet HUD, checklist
+##   "always"    — nodes that never hide (reserved for future use)
+##
+## Usage from any node:
+##   HudGroups.register(self, "gameplay")
+##   HudGroups.show_group("implant")    # shows implant, hides gameplay
+##   HudGroups.show_group("gameplay")   # shows gameplay, hides implant
+##   HudGroups.toggle_group("implant")  # toggle between gameplay and implant
+
+## Which groups are mutually exclusive — showing one hides the others.
+## Modal and debug are independent (they overlay on top of anything).
+const EXCLUSIVE_GROUPS: Array[String] = ["gameplay", "implant"]
+
+var _groups: Dictionary = {}  # group_name -> Array[Control]
+var _active_exclusive: String = "gameplay"
+
+
+func register(node: Control, group: String) -> void:
+	if not _groups.has(group):
+		_groups[group] = []
+	if node not in _groups[group]:
+		_groups[group].append(node)
+	# Apply current visibility state
+	if group in EXCLUSIVE_GROUPS:
+		node.visible = (group == _active_exclusive)
+
+
+func unregister(node: Control, group: String) -> void:
+	if _groups.has(group):
+		_groups[group].erase(node)
+
+
+## Show a group. If it's exclusive, hide the other exclusive groups.
+func show_group(group: String) -> void:
+	if group in EXCLUSIVE_GROUPS:
+		_active_exclusive = group
+		for g: String in EXCLUSIVE_GROUPS:
+			_set_group_visible(g, g == group)
+	else:
+		_set_group_visible(group, true)
+
+
+## Hide a group. If exclusive, falls back to "gameplay".
+func hide_group(group: String) -> void:
+	if group in EXCLUSIVE_GROUPS:
+		show_group("gameplay")
+	else:
+		_set_group_visible(group, false)
+
+
+## Toggle between gameplay and the specified group.
+func toggle_group(group: String) -> void:
+	if _active_exclusive == group:
+		show_group("gameplay")
+	else:
+		show_group(group)
+
+
+## Check if a group is currently active.
+func is_group_active(group: String) -> bool:
+	if group in EXCLUSIVE_GROUPS:
+		return _active_exclusive == group
+	# Non-exclusive: check if any member is visible
+	if _groups.has(group):
+		for node: Control in _groups[group]:
+			if is_instance_valid(node) and node.visible:
+				return true
+	return false
+
+
+func _set_group_visible(group: String, vis: bool) -> void:
+	if not _groups.has(group):
+		return
+	for node: Control in _groups[group]:
+		if is_instance_valid(node):
+			node.visible = vis

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,