fix(ios): keep QR capture work off main thread

[wgqqqqq]

Problem

Scanning a pairing QR code could freeze the iOS UI because AVCaptureSession.startRunning() and stopRunning() executed synchronously on the main thread. The scanner also let UIKit and SwiftUI dismiss the same cover independently.

Changes

• create and operate the capture session on a dedicated serial queue
• deliver QR results to the main thread only after capture stops
• ignore duplicate metadata callbacks
• stop capture when the scanner disappears
• make SwiftUI the single owner of scanner dismissal

Validation

• iPhone 16 Pro simulator Debug build
• signed generic iOS device Debug build
• ./Testing/run-pure-swift-tests.sh
• git diff --check upstream/main...HEAD
• iPhone 13 Pro, iOS 26.3: QR recognition stopped capture off the main thread, pairing requests returned HTTP 200, subsequent 15-second heartbeats succeeded, the process stayed alive, and no crash report was produced

The live device run verified the queue and dismissal fix. The final lazy-initialization refinement was then device-build verified after the log exposed a separate capture-session initialization stall.