fix(client-runtime): settled-only dispatch index carries null callTime; README matches

Responding to ds-review-bot round 2 on #653: the tool/code-dispatch event
is appended at settlement, so using its time as callTime fabricated a
zero-duration call for duration-aware consumers — it is now null (start
unknown) per the ToolResultNode contract, pinned in the session spec. The
README's codeDispatches section described the PR3 running→settled
lifecycle a stack ahead of this tree; it now documents the settled-only
index this PR ships (the running shape lands with the start event in
#658, which already merges cleanly over this).

packages/client/runtime/tests/session.spec.ts

@@ -659,6 +659,9 @@ describe('run_code sub-dispatch indexing', () => {
     expect(subs?.[0]).toMatchObject({
       kind: 'tool-result', callId: 'p1:code:1',
       call: { name: 'bash', argsRaw: '{"command":"ls","description":"列目录"}' },
+      // The settle event carries no start time: callTime stays null (never a
+      // fabricated zero-duration).
+      callTime: null,
       isError: false, content: [{ type: 'text', text: 'demo.txt' }],
     })
     expect(subs?.[1]).toMatchObject({ callId: 'p1:code:2', isError: true })