feat(passkeys_darwin): add SwiftPM support

Adds Package.swift at the Flutter-conventional path so Flutter's plugin
tooling discovers it (FlutterGeneratedPluginSwiftPackage now includes
passkeys_darwin and the plugin is consumed via SwiftPM rather than
falling back to CocoaPods).

- Package.swift at darwin/passkeys_darwin/ (matches shared_preferences_foundation)
- Swift sources moved to darwin/passkeys_darwin/Sources/passkeys_darwin/
- Podspec source_files path and deployment targets aligned with
  Package.swift (iOS 13.0, macOS 10.15); @available runtime gating
  unchanged
- .gitignore for SwiftPM build artifacts
- CLAUDE.md note documenting dual-build (SwiftPM + CocoaPods) convention

Verified on iOS and macOS in both modes: with SwiftPM enabled,
passkeys_darwin appears in the generated SwiftPM graph and drops out of
Podfile.lock; with SwiftPM disabled, the updated podspec source_files
path resolves and pod install pulls the plugin as before.

Closes #234. Closes #235.

Author: kopy

CLAUDE.md

@@ -28,6 +28,7 @@ Melos monorepo. `melos bootstrap` to set up, then `melos run <script>` (see `mel
 - **Web JS changes**: `melos run build-passkeys-web-javascript` (builds TS, copies bundle to example)
 - **Before committing**: `melos run format:check && melos run analyze`
 - **PR titles**: Conventional Commits, e.g. `fix(passkeys_android): fixed a bug!`
+- **`passkeys_darwin` dual-build**: ships both SwiftPM (`darwin/passkeys_darwin/Package.swift`) and CocoaPods (`darwin/passkeys_darwin.podspec`). When changing source layout, paths, or platform minimums, update **both** manifests and verify each path builds: `flutter config --enable-swift-package-manager` then `flutter build ios` (SwiftPM); `--no-enable-swift-package-manager` then `flutter build ios` (pods, must show `Running pod install`). Keep deployment targets aligned between the two; runtime API gating belongs in `@available`, not deployment target.
 
 ## Release order
 

packages/passkeys/passkeys_darwin/.gitignore

@@ -0,0 +1,4 @@
+# SwiftPM
+.build/
+.swiftpm/
+Package.resolved

packages/passkeys/passkeys_darwin/darwin/passkeys_darwin.podspec

@@ -12,7 +12,7 @@ Pod::Spec.new do |s|
   s.license          = { :type => 'BSD', :file => '../LICENSE' }
   s.author           = { 'Your Company' => 'email@example.com' }
   s.source           = { :path => '.' }  
-  s.source_files = 'Classes/**/*'
+  s.source_files = 'passkeys_darwin/Sources/passkeys_darwin/**/*.swift'
 
   # Flutter.framework does not contain a i386 slice.
   s.pod_target_xcconfig = {
@@ -25,6 +25,6 @@ Pod::Spec.new do |s|
 
   s.ios.dependency 'Flutter'
   s.osx.dependency 'FlutterMacOS'
-  s.ios.deployment_target = '9.0'
-  s.osx.deployment_target = '13.5'
+  s.ios.deployment_target = '13.0'
+  s.osx.deployment_target = '10.15'
 end

packages/passkeys/passkeys_darwin/darwin/passkeys_darwin/Package.swift

@@ -0,0 +1,20 @@
+// swift-tools-version: 5.9
+import PackageDescription
+
+let package = Package(
+    name: "passkeys_darwin",
+    platforms: [
+        .iOS("13.0"),
+        .macOS("10.15"),
+    ],
+    products: [
+        .library(name: "passkeys-darwin", targets: ["passkeys_darwin"])
+    ],
+    dependencies: [],
+    targets: [
+        .target(
+            name: "passkeys_darwin",
+            dependencies: []
+        )
+    ]
+)