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.

scripts/sprite-manifest.mjs

@@ -0,0 +1,48 @@
+#!/usr/bin/env node
+// Lists every sprite (all forms, shiny and female variants) with its size, so the offline worker can
+// save the complete set on request and tell whether anything is still missing.
+import path from 'node:path';
+import process from 'node:process';
+import { readdir, stat, writeFile } from 'node:fs/promises';
+import { fileURLToPath } from 'node:url';
+
+export const SPRITE_MANIFEST_NAME = 'manifest.json';
+
+async function walk(dir, files = []) {
+	for (const entry of await readdir(dir, { withFileTypes: true })) {
+		const fullPath = path.join(dir, entry.name);
+		if (entry.isDirectory()) await walk(fullPath, files);
+		else if (entry.isFile() && entry.name.endsWith('.webp')) files.push(fullPath);
+	}
+	return files;
+}
+
+/** Builds the manifest for `<spritesDir>/home`, with paths relative to that folder. */
+export async function buildSpriteManifest(spritesDir) {
+	const homeDir = path.join(spritesDir, 'home');
+	const files = await walk(homeDir);
+	const entries = await Promise.all(
+		files.map(async (file) => [
+			path.relative(homeDir, file).split(path.sep).join('/'),
+			(await stat(file)).size
+		])
+	);
+	entries.sort(([a], [b]) => (a < b ? -1 : a > b ? 1 : 0));
+	return { version: 1, files: entries };
+}
+
+export async function writeSpriteManifest(spritesDir) {
+	const manifest = await buildSpriteManifest(spritesDir);
+	await writeFile(path.join(spritesDir, SPRITE_MANIFEST_NAME), `${JSON.stringify(manifest)}\n`);
+	return manifest;
+}
+
+if (process.argv[1] === fileURLToPath(import.meta.url)) {
+	const spritesDir =
+		process.env.SPRITE_OUTPUT_DIR ?? path.join(process.cwd(), 'static', 'sprites-small');
+	const manifest = await writeSpriteManifest(spritesDir);
+	const bytes = manifest.files.reduce((total, [, size]) => total + size, 0);
+	console.log(
+		`Wrote ${manifest.files.length} sprites (${(bytes / 1048576).toFixed(1)} MB) to ${SPRITE_MANIFEST_NAME}`
+	);
+}