fix(client-runtime): settle-only dispatch windows carry null callTime

Responding to ds-review-bot on #664 (root cause lives here): when a history
window carries a tool/code-dispatch settle without its paired start, the
runtime fabricated callTime = settle time, so downstream duration views
presented a measured 0 ms. Match the native tool-result contract instead —
callTime: null = unknown — and pin it; the trajectory cell already renders
null as the em dash, and the waterfall gains explicit unknown handling in
its own PR.

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

@@ -688,6 +688,9 @@ describe('run_code sub-dispatch indexing', () => {
       isError: false, content: [{ type: 'text', text: 'demo.txt' }],
     })
     expect(subs?.[1]).toMatchObject({ callId: 'p1:code:2', isError: true })
+    // No paired start in the window: duration is UNKNOWN (null), never a
+    // fabricated zero-duration span.
+    expect(subs?.[0]).toMatchObject({ callTime: null })
     // Sub-dispatches never join the surface flow.
     expect(session.getSnapshot().nodes.some(n => n.kind === 'tool-result' && n.callId.includes(':code:'))).toBe(false)
   })