fix(sp25): typecheck fixups

Three small follow-ups after the live-tail triplet wired up:

  * evictOldest is now generic over (K extends string|number, V) so
    the addAck / addTa1Ack call sites don't need a type assertion.
  * Acks.test.tsx no longer spreads importOriginal() — TypeScript
    was choking on the inferred 'unknown' from the mock factory.
    Use the same ApiError / mock shape that Claims.test.tsx uses.
  * tail-stream.test.ts guards the optional gen.return() with
    so strict mode TS2722 stops complaining about possibly-undefined.

src/lib/tail-stream.test.ts

@@ -106,7 +106,7 @@ describe("streamTail", () => {
     // before we close the stream.
     await gen.next();
     driver.close();
-    await gen.return(undefined);
+    await gen.return?.(undefined);
     expect(fetchMock).toHaveBeenCalledTimes(1);
     const [calledUrl] = fetchMock.mock.calls[0] as [string];
     expect(calledUrl).toBe("/api/acks/stream");
@@ -121,7 +121,7 @@ describe("streamTail", () => {
     const gen = streamTail("ta1_acks");
     await gen.next();
     driver.close();
-    await gen.return(undefined);
+    await gen.return?.(undefined);
     expect(fetchMock).toHaveBeenCalledTimes(1);
     const [calledUrl] = fetchMock.mock.calls[0] as [string];
     expect(calledUrl).toBe("/api/ta1-acks/stream");