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/project.godot

@@ -28,6 +28,7 @@ UIStrings="*res://scripts/autoloads/ui_strings.gd"
 FogState="*res://scripts/autoloads/fog_state.gd"
 AudioManager="*res://scripts/autoloads/audio_manager.gd"
 SessionManager="*res://scripts/autoloads/session_manager.gd"
+HudGroups="*res://scripts/autoloads/hud_groups.gd"
 HardwareDetector="*res://ui/hardware_detector.gd"
 
 [audio]