fix(ios): run Talsec.start off the platform thread

[shoaib-akhtar]

Fixes #231

Problem

SwiftFreeraspPlugin handles the start method call on the platform (main) thread and calls Talsec.start(config:) inline. Talsec.start generates a key through SecKeyGeneratePair → SecItemAdd, which is a synchronous XPC round trip to securityd. On iOS 26 that round trip has been measured blocking the main thread for 30+ seconds, which causes app-hang reports and WatchdogTermination kills on first launch (details in #231). Because the work runs inline, the Dart-side await Talsec.instance.start(config) gives no relief.

Change

• Talsec.start now runs on a serial background queue (talsec.app/freerasp/start, .userInitiated).
• The Flutter result is delivered on the main thread after Talsec.start returns, so await Talsec.instance.start(...) still means "the SDK has started".
• Config decoding errors are still returned straight away, as before.
• The other method channel calls are unchanged. blockScreenCapture/isScreenCaptureBlocked already hop to main.

This matches how the React Native bridge already calls Talsec.start: RN module methods run on a background method queue, so running the SDK start off-main has precedent in production.

I went with an explicit queue rather than a channel taskQueue: (option 1 in the issue) so that only start moves off the main thread and the change can be unit-tested directly.

Tests (written first)

The repo had no native iOS tests, so this adds a RunnerTests XCTest target to the example app (example/ios) and a small seam (talsecStarter) so tests can observe the call without running TalsecRuntime.

The commits are ordered red → green:

1. test(ios): …: the tests, run against the unchanged bridge:
   • testStartRunsTalsecOffTheMainThread: fails (Optional(true) is not equal to Optional(false))
   • testStartReturnsBeforeTalsecFinishes: fails (.timedOut, because handle blocked until start finished)
   • testStartRepliesOnTheMainThreadAfterTalsecStarts: passes (guard)
   • testStartWithInvalidConfigReturnsErrorWithoutStartingTalsec: passes (guard)
2. fix(ios): …: all 4 pass.

Run with:

cd example && flutter pub get && cd ios && pod install
xcodebuild test -workspace Runner.xcworkspace -scheme Runner \
  -destination 'platform=iOS Simulator,name=iPhone 17 Pro' -only-testing:RunnerTests

Verified on Xcode 26.1.1, iOS 26.1 simulator, Flutter 3.38.2. As an extra check, I also called the real TalsecRuntime start through the patched plugin from a test. It started off-main, replied, and did not crash.

Not covered

The issue also shows the same keychain path being reached from inside TalsecRuntime through a UIScreen KVO/notification callback. That code lives in the closed-source xcframework, so this PR can't change it.