refactor(anki): parse the known-word cache in one shared module

The cache manager and the stats server each carried a hand-written parser
for the same file. When the format went to v4 for maturity tiers only the
manager was updated, so loadKnownWordsSet fell through to null and every
session reported 0 known words from an intact cache.

Move the state union, its parser, and a knownWordsFromState accessor into
known-word-cache-format and have both readers go through it. The stats
server no longer mentions a version at all, and both version switches close
with assertNever, so adding a V5 is a compile error at every consumer
instead of a silently empty result. An unparseable cache now warns rather
than reading as "no known words".

Two stats fixtures omitted refreshedAtMs/scope and only passed because the
stats reader was laxer than the manager; they now carry the fields every
cache the app writes has always had.

src/anki-integration/known-word-cache-format.test.ts

@@ -0,0 +1,76 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+
+import {
+  KnownWordCacheState,
+  knownWordsFromState,
+  parseKnownWordCacheState,
+} from './known-word-cache-format';
+
+function parseOrThrow(value: unknown): KnownWordCacheState {
+  const parsed = parseKnownWordCacheState(value);
+  assert.ok(parsed, 'expected the payload to parse');
+  return parsed;
+}
+
+const BASE = { refreshedAtMs: 1, scope: 'deck:test' };
+
+test('known-word cache format reads words from every version the union covers', () => {
+  assert.deepEqual(
+    knownWordsFromState(parseOrThrow({ ...BASE, version: 1, words: ['する'] })),
+    new Set(['する']),
+  );
+
+  assert.deepEqual(
+    knownWordsFromState(
+      parseOrThrow({ ...BASE, version: 2, words: ['する'], notes: { '1': ['する'] } }),
+    ),
+    new Set(['する']),
+  );
+
+  assert.deepEqual(
+    knownWordsFromState(
+      parseOrThrow({
+        ...BASE,
+        version: 3,
+        notes: { '1': [{ word: 'する', reading: 'する' }], '2': [{ word: '猫', reading: null }] },
+      }),
+    ),
+    new Set(['する', '猫']),
+  );
+
+  // v4 only adds `tiers`; the words a reader sees must not change.
+  assert.deepEqual(
+    knownWordsFromState(
+      parseOrThrow({
+        ...BASE,
+        version: 4,
+        notes: { '1': [{ word: 'する', reading: 'する' }], '2': [{ word: '猫', reading: null }] },
+        tiers: { '1': 'mature', '2': 'young' },
+      }),
+    ),
+    new Set(['する', '猫']),
+  );
+});
+
+test('known-word cache format rejects payloads that are not a known cache state', () => {
+  const notes = { '1': [{ word: 'する', reading: null }] };
+
+  assert.equal(parseKnownWordCacheState(null), null);
+  assert.equal(parseKnownWordCacheState('{}'), null);
+
+  // An unknown version must not be read as an empty cache.
+  assert.equal(parseKnownWordCacheState({ ...BASE, version: 99, notes }), null);
+
+  assert.equal(
+    parseKnownWordCacheState({ version: 4, scope: 'deck:test', notes, tiers: {} }),
+    null,
+  );
+  assert.equal(parseKnownWordCacheState({ version: 4, refreshedAtMs: 1, notes, tiers: {} }), null);
+
+  // v4 without its tiers map is a v3 payload mislabelled as v4.
+  assert.equal(parseKnownWordCacheState({ ...BASE, version: 4, notes }), null);
+
+  // v3 carries entry objects, not the bare strings v2 used.
+  assert.equal(parseKnownWordCacheState({ ...BASE, version: 3, notes: { '1': ['する'] } }), null);
+});