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

src/test/tests/test_ui_art.gd

@@ -0,0 +1,38 @@
+extends RefCounted
+## Headless tests for the runtime image loader (ADR-0014).
+
+const UiArt := preload("res://src/game/ui/art.gd")
+
+func test_loads_runtime_ui_art() -> bool:
+	var art := UiArt.new()
+	var start := art.texture("start_menu")
+	var venue := art.texture("venue_overview")
+	var a := TestUtils.check(start != null, "start_menu art loads from assets/images/ui")
+	var b := TestUtils.check(venue != null, "venue_overview art loads from assets/images/ui")
+	var c := TestUtils.check(art.has("start_menu"), "has() sees the runtime art")
+	return a and b and c
+
+func test_missing_art_returns_null() -> bool:
+	var art := UiArt.new()
+	var missing := art.texture("does_not_exist")
+	var empty := art.texture("")
+	var a := TestUtils.check(missing == null, "missing art returns null for a fallback")
+	var b := TestUtils.check(empty == null, "empty name returns null")
+	return a and b
+
+func test_explicit_relative_path() -> bool:
+	var art := UiArt.new()
+	var texture := art.texture_at("ui/start_menu.png")
+	var missing := art.texture_at("ui/nope.png")
+	var a := TestUtils.check(texture != null, "texture_at loads an explicit path")
+	var b := TestUtils.check(missing == null, "texture_at returns null when absent")
+	return a and b
+
+func test_cache_returns_the_same_resource() -> bool:
+	var art := UiArt.new()
+	var first := art.texture("start_menu")
+	var second := art.texture("start_menu")
+	var a := TestUtils.check(first != null and first == second, "textures are cached")
+	art.clear_cache()
+	var b := TestUtils.check(art.texture("start_menu") != null, "cache can be cleared")
+	return a and b