perf(assets): ship key art as webp and drop unreferenced media

Fifty-seven percent of the built output was reachable by nothing but a
manifest that is never fetched at runtime. The key art documented as the
menu background was never wired up; a 1.2KB placeholder was loaded instead.

- wire key art into the menu, ending screen and act four to six intros
- re-encode key art to webp: 1.63MB to 556KB at DSSIM around 0.012
- move 3.9MB of unwired voice lines and authoring sources out of the
  bundle into unshipped-assets, still tracked rather than deleted
- add 192 and 512 icons, the sizes an install prompt requires

src/lib/generated-assets.js

@@ -55,7 +55,31 @@ export const GENERATED_SCENES = [
     `${BASE}scene-chamber.png`,
 ];
 
+// Hand-authored 1280x720 key art, stored as webp (the source pngs were ~3x the
+// size at no visible gain on a backdrop rendered under a gradient).
+const KEYART_BASE = 'assets/images/';
+
+export const KEYART = {
+    garden: `${KEYART_BASE}keyart-garden.webp`,
+    gameover: `${KEYART_BASE}keyart-gameover.webp`,
+    underground: `${KEYART_BASE}keyart-underground.webp`,
+    palace: `${KEYART_BASE}keyart-palace.webp`,
+    throne: `${KEYART_BASE}keyart-throne.webp`,
+};
+
+// Level-intro backdrops. Acts 4-6 use the detailed key art; earlier acts use the
+// generated scene tiles, which suit the simpler outdoor levels.
+const KEYART_BY_LEVEL = {
+    7: KEYART.underground,
+    8: KEYART.underground,
+    9: KEYART.palace,
+    10: KEYART.palace,
+    11: KEYART.palace,
+    12: KEYART.throne,
+};
+
 export function generatedSceneForLevel(level) {
+    if (KEYART_BY_LEVEL[level]) return KEYART_BY_LEVEL[level];
     const idx = Math.max(0, Math.min(GENERATED_SCENES.length - 1, level - 1));
     return GENERATED_SCENES[idx];
 }