perf(sprites): ship 192px sprites and a manifest of every sprite

Sprites were shipped at 512x512 but render at 44-64px in the box grid.
Resizing to 192px halves each file (median 15.5KB -> 7.6KB); a full dex
of artwork drops from 16.6MB to 7.6MB.

The sprite build now also writes static/sprites-small/manifest.json,
listing every sprite (all forms, shiny and female variants) with its
size, so the offline worker can save the complete set and report what
is missing. A unit test keeps it in step with the files on disk.

tests/unit/spriteManifest.test.ts

@@ -0,0 +1,35 @@
+import { readFileSync } from 'node:fs';
+import path from 'node:path';
+import { describe, expect, it } from 'vitest';
+// @ts-expect-error - plain ESM build script without type declarations
+import { buildSpriteManifest } from '../../scripts/sprite-manifest.mjs';
+import { resolveSpriteUrl, spriteRoot } from '$lib/utils/spriteUrl';
+
+const spritesDir = path.join(process.cwd(), 'static', 'sprites-small');
+
+describe('sprite manifest', () => {
+	const committed = JSON.parse(readFileSync(path.join(spritesDir, 'manifest.json'), 'utf8'));
+
+	it('lists exactly the sprite files on disk, with their sizes', async () => {
+		// "Save all artwork" downloads this list, so a stale manifest would miss or 404 sprites.
+		expect(committed).toEqual(await buildSpriteManifest(spritesDir));
+	});
+
+	it('covers forms, shiny and female variants', () => {
+		const files: string[] = committed.files.map(([file]: [string, number]) => file);
+		expect(files.some((file) => file.startsWith('shiny/'))).toBe(true);
+		expect(files.some((file) => file.startsWith('female/'))).toBe(true);
+		expect(files.some((file) => file.startsWith('shiny/female/'))).toBe(true);
+		// Forms use either a "<number>-<form>" key or a 10000+ national-style number.
+		expect(files.some((file) => /^\d+-[a-z0-9-]+\.webp$/.test(file))).toBe(true);
+		expect(files.some((file) => /^10\d{3}\.webp$/.test(file))).toBe(true);
+	});
+
+	it('uses the same folder that sprite URLs resolve into', () => {
+		for (const useLocal of [true, false]) {
+			expect(
+				resolveSpriteUrl({ pokedexNumber: 25, form: '', spriteKey: '25' }, true, useLocal)
+			).toBe(`${spriteRoot(useLocal)}/shiny/25.webp`);
+		}
+	});
+});