Release an iOS xcframework SDK; release 0.7.1

[adamski]

Adds an iOS prebuilt SDK to the release, so iOS apps can link a released libreplicant_client instead of a local cargo build.

Changes

• release.yml: the macOS job also builds aarch64-apple-ios and aarch64-apple-ios-sim (IPHONEOS_DEPLOYMENT_TARGET=16.0). It packages them as a static libreplicant_client.xcframework (slices ios-arm64 and ios-arm64-simulator) with the usual headers, in replicant-sdk-<version>-ios-xcframework.tar.gz.
• release.yml: adds workflow_dispatch for dry runs. A manual run uploads artifacts but doesn't publish a release (create-release runs only on tag pushes). Archive names use SDK_VERSION: the tag name for tag builds, as before, and dev-<sha> for manual runs.
• FetchReplicant.cmake: on CMAKE_SYSTEM_NAME=iOS it selects the xcframework asset and links it into replicant_client. It needs CMake 3.28+ on iOS, supports static linking only, and gives a clear error for releases without an iOS asset (the first ones with it are 0.6.5 and 0.7.1). Behaviour on macOS, Linux and Windows is unchanged.
• Releases 0.7.1. The same CI change is on release/0.6 for a 0.6.5 backport.

Simulator note

CMake picks the xcframework slice from CMAKE_OSX_SYSROOT at configure time. To switch device and simulator with xcodebuild -sdk on one project, set CMAKE_XCODE_LINK_BUILD_PHASE_MODE=KNOWN_LOCATION. There is no x86_64 simulator slice.

Verification

• Device and simulator Release builds succeed locally for 0.7.1 and 0.6.5. All slices are thin arm64, platform 2 (device) or 7 (simulator), minos 16.0.
• A throwaway iOS CMake project (Xcode generator) links through the real FetchReplicant.cmake download and extract path, for device and arm64 simulator, on both versions.
• The macOS fetch still returns the universal asset with an unchanged hash.
• actionlint shows only the existing softprops/action-gh-release@v1 warning.

Only CI can verify the GitHub job and the release upload. After merge, run gh workflow run release.yml --ref main and check the replicant-sdk-macos artifact before tagging v0.7.1.