feat: keep tool calls working for a short time after backgrounding on iOS (#139)

[V3RON]

Closes #139

What changed

Pending. Red tests for the iOS background window: the client holds OS background time after backgrounding and closes with closeForBackground() only when the time expires.

Acceptance criteria

#	Criterion	Test	Tier
1	Backgrounding holds time, socket stays open, a call is answered	testBackgroundingAnActiveSessionHoldsBackgroundTimeAndKeepsTheSocketOpen, testAToolCallArrivingWhileBackgroundTimeIsHeldIsAnsweredOnTheSameSocket	unit (Swift)
2	In-flight call completes while time is held	testACallInFlightWhenTheAppIsBackgroundedCompletesWhileBackgroundTimeIsHeld	unit (Swift)
3	Expiry closes for background, cancels in-flight, no reconnect, time ends after the close event, foreground resumes	testWhenBackgroundTimeExpires... (2), testBackgroundTimeEndsOnlyOnceTheCloseEventHasArrived, testWindowEndCloseEmitsNoErrorEvent, testForegroundingAfterTheWindowEndsResumesTheSession	unit (Swift)
4	Foreground before expiry ends time, same socket	testForegroundingBeforeTheWindowEndsEndsTheBackgroundTimeAndKeepsTheSameSocket	unit (Swift)
5	No session asks for no time; disconnect, destroy, lost socket end it	testBackgroundingWithoutAnActiveSessionAsksForNoBackgroundTime, testDisconnectEndsTheBackgroundTime, testDestroyEndsTheBackgroundTime, testALostSocketEndsTheBackgroundTime	unit (Swift)
6	Device E2E on the simulator	pending	device
7	Docs and changelog	pending	n/a

E2E evidence

pending

Checklist

• CHANGELOG.md has an entry under Unreleased (writing-changelog skill), or the change is not user-visible
• User-facing docs updated for every surface the change touches (writing-user-docs skill), or the change is not user-visible
• No new import past a module's index.ts; no new direct node:* I/O outside an adapter
• Simplification checklist from the architecture skill applied, exceptions explained above
• docs/ARCHITECTURE.md updated if a surface it describes changed

Out of scope

none

Status

Implement: red tests committed Review: pending E2E: pending Ready: no