review: address ds-review-bot findings on session-stats

- stateVersion starts at 1: the package is new, no persisted rows predate it
- pendingCalls pairs by own key so a provider callId naming a prototype
  property cannot fold toolMs to NaN on an unmatched crash-recovery result
- StatsLine folds the window fallback only when no sessionStats value is
  served, and gates the token group on actual token activity instead of
  steps, so failed-only sessions drop the zero-token group
- correct the crash-step counting semantics in the README and Agent Note:
  recovery closes interrupted steps with a synthetic step/end on reload
- reword the window-scoped alternative as a plain rejected option and name
  isTokenDelta's home beside the StreamChunk type

apps/web/tests/math-rendering.e2e.ts

@@ -120,7 +120,7 @@ describe('web e2e: settled Markdown math rendering', () => {
     await expect.poll(() => page.locator('.katex-display').count(), { timeout: 10_000 }).toBe(2)
     expect(await page.locator('.katex-error').count()).toBe(0)
     await expect.poll(
-      () => page.getByText('Input 0 tok · Output 0 tok', { exact: false }).count(),
+      () => page.getByText('1 turns · 1 steps', { exact: false }).count(),
       { timeout: 10_000 },
     ).toBe(1)