Fix background crash in Coach BGTask launch handler - #24

What

Fixes a periodic background crash (EXC_BREAKPOINT / SIGTRAP) in the Coach daily check-in background task.

CoachNotificationScheduler.register(...) registers the BGTask launch handler with MainActor.assumeIsolated { ... }, on the assumption that BGTaskScheduler.register(forTaskWithIdentifier:using:) with queue: nil runs the handler on the main queue.

It does not — nil means a private background queue. assumeIsolated is a precondition assertion (not a thread hop): when the handler runs off the main actor, the assertion fails and Swift traps, crashing the app every time iOS opportunistically fires the com.pulseloop.coach.refresh task in the background.

Fix

Hop to the main actor explicitly instead of asserting we're already on it:

BGTaskScheduler.shared.register(forTaskWithIdentifier: Self.taskIdentifier, using: nil) { task in
    Task { @MainActor in Self.shared.handle(task) }
}

handle(_:) and the @MainActor-isolated state it touches now genuinely run on the main actor. The async hop is well within the BGTask runtime budget; scheduleNext() / setTaskCompleted behavior is unchanged.

How it was found

Captured in a TestFlight crash report. The crashing thread was a libdispatch background worker:

Thread 5 Crashed:
0  libdispatch.dylib   _dispatch_assert_queue_fail
3  libswift_Concurrency.dylib  _swift_task_checkIsolatedSwift
5  PulseLoop  MainActor.assumeIsolated<A>(_:file:line:)
6  PulseLoop  closure #1 in CoachNotificationScheduler.register(serviceProvider:)  (CoachNotificationScheduler.swift:19)
7  PulseLoop  thunk for @escaping @callee_guaranteed (BGTask) -> ()
8  BackgroundTasks  __41-[BGTaskScheduler _runTask:registration:]_block_invoke_5

The symptom matched exactly: crashes only in the background, never during active use.

Testing

• Clean Debug simulator build (iPhone 17 Pro, iOS 26.5).
• The handler can be exercised on a debug build via LLDB:
  e -l objc -- (void)[[BGTaskScheduler sharedScheduler] _simulateLaunchForTaskWithIdentifier:@"com.pulseloop.coach.refresh"]

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