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

tools/screen_designs.py

@@ -9,7 +9,7 @@ screenshots against.
 
 Two commands:
 
-  generate   write one reference image per screen to docs/screens/designs/.
+  generate   write one runtime reference image per screen to assets/images/ui/.
              Two modes:
                --ai     realistic 3D game scenes and in-game screen designs via
                         the OpenRouter image API (default; requires
@@ -23,7 +23,7 @@ Two commands:
              Requires OPENROUTER_API_KEY (falls back to the repo .env).
                --dry-run  build and print the requests without calling the API.
 
-  compare    compare docs/screenshots/*.png against the reference designs and
+  compare    compare docs/screenshots/*.png against the runtime reference art and
              flag gross drift. Semantic composition is judged with coarse,
              Pillow-only signals (aspect ratio, frame size, blank-frame
              contrast, grayscale MAE, colour-histogram distance, edge density).
@@ -53,7 +53,9 @@ from PIL import Image, ImageChops, ImageDraw, ImageFilter, ImageOps, ImageStat
 import generate_textures
 
 ROOT = Path(__file__).resolve().parent.parent
-DESIGNS_DIR = ROOT / "docs" / "screens" / "designs"
+## Runtime reference art (ADR-0014); docs/screens/designs/ is the reference-only
+## snapshot kept for docs and historical comparison.
+DESIGNS_DIR = ROOT / "assets" / "images" / "ui"
 SCREENSHOTS_DIR = ROOT / "docs" / "screenshots"
 
 # Reference "design" dimensions — match the capture tool and the README gallery.
@@ -1022,7 +1024,7 @@ def build_parser():
     commands = parser.add_subparsers(dest="command", required=True)
 
     gen = commands.add_parser(
-        "generate", help="write reference design images to docs/screens/designs/")
+        "generate", help="write reference design images to assets/images/ui/")
     mode = gen.add_mutually_exclusive_group()
     mode.add_argument("--draft", action="store_true",
                       help="draw deterministic wireframes (no API/network)")