Update OOM handling

[melekr]

WHY

To fix an issue where app‑launch stalls when OOM detection is enabled. Caused by heavy plcr snapshots and sync operations.

This PR:

• Offloads all OOM and PLCR I/O to a dedicated serial DispatchQueue.
• Provides a 3 options self‑documenting explicit switch (.none | .light | .full) for OOM reporting.
• Eliminates launch stalls by moving all OOM work to a dedicated serial queue and introducing a lightweight snapshot mode that skips symbolication.
• Provides automatic fallback (light → full) to guarantee that an OOM is still reported even if the lightweight path fails.
• Maintains backwards compatibility and adds deprecation & compiler notices.
• Adds resident memory footprint at the moment of the low‑memory warning for easier triage.

WHAT CHANGED

API

• BacktraceClientConfiguration gains oomMode: BacktraceOomMode.
• Legacy detectOom Bool is deprecated but still functional.
• No other public signatures changed.

Runtime

• BacktraceOomWatcher executes on DispatchQueue(label: "com.backtrace.oom").
• New lightweight report path (current thread, no symbolication).
• Normal termination now early‑exits when mode == .none.

Tests

• Added flushQueue() helper and updated specs.
• New coverage for .none and .light modes.

Sample Reports

• .full
• .light

WHAT'S NEXT [Swift 6 Concurrency 🙌]

Grand Central Dispatch (DispatchQueue) is the established concurrency framework that predates the modern Swift concurrency. It solves the immediate problem and is fully compatible & safe in a Swift 6 environment.
A more idiomatic Swift 6 approach however, would use async/await and Actors.

actor BacktraceOomWatcher {

    // actor guarantees concurrency
    internal var state: ApplicationInfo
    var quietTimeInMillis: Int = 60 * 1000

    // oomQueue is no longer needed.

    init(...) {
        // standard initialization
    }

    // This function is now 'async'. The 'actor' ensures it runs serially.
    func start() async {
        await sendPendingOomReports()
        ...
    }

    // Callers from outside the actor must 'await' this call.
    func handleLowMemoryWarning() async {
        ...
        await saveState()
    }

    // private methods are also isolated to the actor.
    private func saveState() {
        ...
    }
    
    private func reportOom(appState: ApplicationInfo) async {
        // generate report
        try? await backtraceApi.send(report)
    }
 }

Usage:

Task {
     await self.backtraceOomWatcher.start()
  }

ref: BT-5903