Move updaters onto public and private storage

Updaters take a storage object instead of touching the filesystem: a
publicStorage (what the site serves: data/, assets/) and a privateStorage
(updater state). Two implementations share one small interface
(exists / readJson / writeJson / readBytes / writeText / writeBytes):

- FilesystemStorage over a directory, used by npm run splatnet, so dist/
  and storage/ end up exactly as they always have and the S3 sync is
  untouched;
- BucketStorage over an R2 bucket binding (MemoryBucket in tests), with a
  thin per-run cache: exists() lists a directory once and answers from
  memory, readJson() parses a document once, and writeJson() skips the put
  when the content is unchanged.

The updater base class and localization processor keep their original
structure with awaits added; the processor reads its document lazily and
writes once per update, as splatoon3.ink's does. Objects written to a
bucket carry the content type and cache-control the S3 sync used to apply.

Tests cover the storage contract (run against both implementations), the
BucketStorage cache behaviour, MemoryBucket's R2-style listing, the
localization processor, the updater base, and each concrete updater.
scripts/compare-data.mjs compares two data directories ignoring SplatNet's
random ordering and ICS timestamps.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

test/storage/MemoryBucket.test.js

@@ -0,0 +1,35 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { MemoryBucket } from '../../src/common/storage/index.js';
+
+// MemoryBucket stands in for the R2 binding in tests, so its list() must paginate like R2's.
+test('list filters by prefix, sorts, and paginates with cursors', async () => {
+  const bucket = new MemoryBucket;
+  for (const key of ['assets/images/stage/b.png', 'assets/images/stage/a.png', 'assets/images/stage/deep/c.png', 'assets/images/weapon/w.png', 'data/x.json'])
+    await bucket.put(key, 'x');
+
+  const all = await bucket.list({ prefix: 'assets/images/stage/' });
+  assert.deepEqual(all.objects.map(o => o.key), ['assets/images/stage/a.png', 'assets/images/stage/b.png', 'assets/images/stage/deep/c.png']);
+  assert.equal(all.truncated, false);
+
+  const first = await bucket.list({ prefix: 'assets/', limit: 2 });
+  assert.equal(first.truncated, true);
+  const second = await bucket.list({ prefix: 'assets/', limit: 2, cursor: first.cursor });
+  assert.deepEqual([...first.objects, ...second.objects].map(o => o.key).length, 4);
+  assert.equal(second.truncated, false);
+});
+
+test('put and get round-trip strings, bytes, and streams with metadata', async () => {
+  const bucket = new MemoryBucket;
+  await bucket.put('a.json', '{"a":1}', { httpMetadata: { contentType: 'application/json' } });
+  await bucket.put('b.png', new Uint8Array([1, 2]));
+  await bucket.put('c.png', new Response(new Uint8Array([3])).body);
+
+  assert.deepEqual(await (await bucket.get('a.json')).json(), { a: 1 });
+  assert.equal((await bucket.head('a.json')).httpMetadata.contentType, 'application/json');
+  assert.deepEqual(new Uint8Array(await (await bucket.get('b.png')).arrayBuffer()), new Uint8Array([1, 2]));
+  assert.deepEqual(new Uint8Array(await (await bucket.get('c.png')).arrayBuffer()), new Uint8Array([3]));
+  assert.equal(await bucket.get('missing'), null);
+  await bucket.delete('a.json');
+  assert.equal(await bucket.head('a.json'), null);
+});