Darwin: stop bundling Flutter framework; delegate to host Xcode project

[lhoward]

Summary

Removes two Darwin-specific assumptions baked into `Package.swift` that broke under common configurations:

1. `/opt/flutter` was hardcoded as the macOS Flutter SDK root — broke any developer whose SDK lived elsewhere, and was useless on Apple Silicon (`darwin-x64` artifact name only).
2. `Flutter.xcframework.zip` (~63 MB) was committed in the repo and wired in as a `.binaryTarget` for iOS — pinned to whatever engine revision the zip was built against, bloated git history, and risked conflicting with the `Flutter.framework` the host Xcode project already provides.

Both are removed. On Darwin, `Flutter.framework` / `FlutterMacOS.framework` is now sourced from the host Xcode project's `$(FRAMEWORK_SEARCH_PATHS)`, populated by Flutter's standard `xcode_backend.sh` / `macos_assemble.sh` build phases — the same mechanism Flutter's SwiftPM-aware plugins (e.g. `url_launcher_macos`) rely on. SPM targets compiled within an Xcode build inherit those search paths, so `import FlutterMacOS` / `import Flutter` continues to resolve in `FlutterPlatformMessenger.swift`.

Linux and Android (cross-compiled on macOS) code paths are untouched: the Linux branch still emits its own engine linker flags, and the Android branch still overwrites `FlutterPlatform` / `FlutterUnsafeLinkerFlags` downstream of the Darwin block.

Tradeoff

Standalone `swift build` on Darwin no longer pulls in the Flutter framework — `FlutterPlatformMessenger` is hidden by `#if canImport(...)`. This is acceptable because the example apps (and intended downstream consumers) reference FlutterSwift via `XCLocalSwiftPackageReference` (path-based), and Xcode always supplies the framework. README updated to call this out.

Test plan

• macOS: `swift test` — 28/28 pass
• macOS counter: `flutter build macos --debug` — built
• iOS counter (simulator): `flutter build ios --debug --no-codesign --simulator` — built
• Android counter: `JAVA_HOME=… ./gradlew :app:assembleDebug` — `BUILD SUCCESSFUL`
• `swift package describe` with `FLUTTER_SWIFT_JVM=true` (Android-on-macOS resolve path)
• eLinux GBM: `./build-elinux.sh` (untouched code, untested locally)
• eLinux Wayland: `FLUTTER_SWIFT_BACKEND=wayland ./build-elinux.sh` (untouched code, untested locally)

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