docs(architecture): update CLAUDE.md, project structure, D-170 for implant UI system

- CLAUDE.md: added Implant UI component library section (D-169, D-170)
- project-structure.md: added client/ui/implant/, hud_groups.gd, gameplay_renderer.gd
- D-170: corrected implementation note from visibility toggling to z-index layering

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

decisions/architecture.md

@@ -638,7 +638,7 @@ Technical foundation decisions that constrain implementation: engine, client-ser
   - `debug` — debug overlay, gauntlet HUD, checklist. Independent.
 - **Exclusivity rules:** Opening any `implant/*` app hides gameplay and any other implant app. Closing an app returns to gameplay. Modal and debug are independent layers.
 - **API:** `HudGroups.register(node, "implant/map")`, `HudGroups.open_app("implant/map")`, `HudGroups.close_app()`, `HudGroups.toggle_app("implant/map")`, `HudGroups.is_app_active("implant/map")`, `HudGroups.is_implant_active()`
-- **Implementation:** `client/scripts/autoloads/hud_groups.gd` — lightweight autoload, no scene tree manipulation beyond `node.visible`.
+- **Implementation:** `client/scripts/autoloads/hud_groups.gd` — lightweight autoload. Uses z-index layer management (not visibility toggling) so all nodes stay active in the tree. Emits `gameplay_occluded` signal when a fullscreen app covers gameplay — renderers extend `GameplayRenderer` base class to pause automatically.
 - **Raised by:** Jeroen, 2026-04-05 — "Walk" badge visible over fullscreen star map.
 - **Dissent:** None.