Start empty when the cache directory holds another account's cache (closes #104)

When metadata.json in the cache directory was written for a different,
non-zero user ID than the client's, Library.open deletes it and mldata/
and loads an empty store, so the first refresh enumerates from 0 and none
of the other account's collections, files, keys or ML results are served.
Only reachable with --cache-dir or an explicit cacheDirectory.

Model: opus-5-5

test/library/mldata.test.ts

@@ -13,6 +13,8 @@
  *   2. `Library` wiring: after each refresh the library fetches ML data through
  *      the metadata pool for every known file not yet cached, is incremental on
  *      later refreshes, and refetches a file whose `updationTime` advanced.
+ *      Opening a cache directory written by another account starts empty,
+ *      its ML data included (issue #104).
  *
  * Embedding values are chosen to be exactly representable as float32 so the
  * round-trip through `clip.f32` compares equal.
@@ -498,4 +500,63 @@ describe("Library ML-data fetch on refresh", () => {
             await lib.close();
         }
     });
+
+    it("starts empty when the cache directory holds another account's cache", async () => {
+        // Account A fills the cache directory: metadata and ML data.
+        const clientA = new MLMockClient();
+        clientA.collectionsQueue.push({
+            collections: [collection(1, 100)],
+            deleted: [],
+            cursor: 100,
+        });
+        clientA.filesFor(1, {
+            files: [file(1001, 1, 90)],
+            deleted: [],
+            cursor: 90,
+        });
+        clientA.mlByFile.set(1001, payload([0.5, 0.25, 0.75]));
+        const libA = await Library.open({
+            client: clientA,
+            cacheDirectory,
+            refreshIntervalSeconds: 3600,
+        });
+        try {
+            await vi.waitFor(
+                () => expect(libA.status().lastMLFetchAt).toBeGreaterThan(0),
+                { timeout: 2000, interval: 5 },
+            );
+        } finally {
+            await libA.close();
+        }
+
+        // Account B opens the same directory.
+        const clientB = new MLMockClient();
+        clientB.userID = USER_ID + 1;
+        const sinceTimes: number[] = [];
+        const realCollectionsSince = clientB.collectionsSince.bind(clientB);
+        clientB.collectionsSince = async (args) => {
+            sinceTimes.push(args.sinceTime);
+            return realCollectionsSince(args);
+        };
+        const libB = await Library.open({
+            client: clientB,
+            cacheDirectory,
+            refreshIntervalSeconds: 3600,
+        });
+        try {
+            expect(sinceTimes[0]).toBe(0);
+            expect(libB.status().userID).toBe(USER_ID + 1);
+            expect(libB.listCollections()).toEqual([]);
+            expect(libB.getFile(1, 1001)).toBeUndefined();
+            expect(await libB.mldata.forFile({ fileID: 1001 })).toBeUndefined();
+            expect(
+                libB.mldata.searchByEmbedding({ embedding: [0.5, 0.25, 0.75] }),
+            ).toEqual([]);
+            expect(
+                existsSync(join(cacheDirectory, "mldata", "1001.json")),
+            ).toBe(false);
+        } finally {
+            await libB.close();
+        }
+    });
 });