fix(macOS): offload compression off the AppKit main thread

[AlexV525]

Summary

FlutterMethodChannel dispatches handle(_:result:) on the platform thread — on macOS that's the AppKit main / UI thread. The plugin ran decode, CGContext resize+rotate, CGImageDestination encode and disk writes inline on that thread, so compressing anything nontrivial stalled the Flutter engine and froze the UI (dropped frames, gesture/input hangs, animation stutter).

Android and iOS already offload:

• Android — every compress method is threadPool.execute { … } on a fixed 8-thread ExecutorService, with results posted back via Handler(Looper.getMainLooper()) (ResultHandler.kt).
• iOS — ImageCompressPlugin.handleMethodCall: wraps the switch in dispatch_async(dispatch_get_global_queue(0, 0), …) (ImageCompressPlugin.m).

macOS now matches: compression runs on DispatchQueue.global(qos: .userInitiated), FlutterResult is marshaled back to the main thread via a small mainResult shim.

What changed

FlutterImageCompressMacosPlugin.swift — handle(_:result:) only:

1. Threading — the three compress branches (compressAndGetFile, compressWithFile, compressWithList) plus default (FlutterMethodNotImplemented) run inside DispatchQueue.global(qos: .userInitiated).async { … }.
2. mainResult shim — wraps FlutterResult so every downstream callback (handleResult's error paths at lines 108/124/132, Compressor.compressToPath / compressToBytes at lines 402/423) always resolves on the main thread regardless of where it's invoked from.
3. showLog reply gap — the previous switch case was Logger.showLog(show: args as! Bool) with no result(…) call, leaving the awaiting Dart future pending. Now returns 1 to match Android (result.success(1)) and iOS (result(@1)). Kept synchronous on the calling thread — it's a fast setter, no reason to hop.
4. [weak self] nil branch — if the plugin is detached before the async block starts, previously the guard let self else { return } swallowed the call silently. Now replies with FlutterError(code: "plugin_detached", …) so the Dart future always resolves.

Also updates packages/flutter_image_compress_macos/macos/.gitignore to ignore SwiftPM's .build/ output that IDE indexing / swift build leaves under macos/flutter_image_compress_macos/.

Test plan

• flutter build macos --debug from the example — clean.
• flutter test integration_test/compress_test.dart -d macos — all 13 macOS-relevant tests pass (7 skipped are Android/iOS-only), including HEIC round-trip, EXIF preservation across JPEG/PNG/HEIC, rotate 90/180/270, transparent PNG, WebP interop.
• Reviewer manual UI-freeze repro (optional): before this PR, compressing a ~20MB image on macOS visibly hitches Flutter animations for the duration of the compress; after this PR, animations run smoothly and the compress just resolves out of band.

Notes for melos changelog

Single fix(macOS): … commit — will land under flutter_image_compress_macos's Bug Fixes section.