Move screen designs to the runtime root and add the UI art loader

- screen_designs.py now reads/writes assets/images/ui/ (ADR-0014); docs/
  screens/designs/ stays the reference-only snapshot, documented as historical
- add src/game/ui/art.gd: cached loader for assets/images/ui|scenes with null
  fallbacks for missing files, plus headless tests
- start menu uses the start_menu scene art as a full-screen backdrop, covers the
  baked-in mockup panel with an opaque panel, and styles the venue rows to match
- update README/architecture/TODO path notes and mark the storage/code-path/loader
  items done; DONE records the historical path move
- compare still 11 good / 0 bad; headless 226/226

README/PLAN realism-pass edits from another workstream remain unstaged." && git log --oneline -1

DONE.md

@@ -887,3 +887,10 @@ artifacts (Linux, Windows, macOS, NSIS installer, AppImage, Web) for tag `v0.6.0
 - [x] Bumped the version and synchronised README/architecture/blender-assets/plan docs.
 - [x] Headless **222/222**, Python **69/69**, all smokes, `compare` **11 good / 0 bad**.
 - [x] Release verified: `https://git.allucanget.biz/zwitschi/security-game/releases/tag/v0.6.0`.
+
+## Runtime Image Path Migration
+
+Completed 2026-11-01. Historical note: the screen-design references originally lived only in
+`docs/screens/designs/`; they were promoted to the runtime root `assets/images/ui/` under
+ADR-0014. `docs/screens/designs/` remains a reference-only snapshot; `tools/screen_designs.py`
+now reads and writes `assets/images/ui/`.