Use xcodebuild for Swift XCFramework assembly

[i386]

Summary

Follow-up to #665. The Swift SDK packaging scripts now let xcodebuild -create-xcframework write the XCFramework structure instead of carrying a shell fallback that hand-writes Info.plist and library metadata.

This PR depends on #665's inner framework-bundle layout fix: xcodebuild -create-xcframework wraps the input frameworks, but it does not validate or repair a flat macOS .framework. Flat-in would still be flat-out. The release verifier now checks that every macOS slice has the required Versions/A layout and canonical symlinks before it runs a consumer smoke.

The scripts still wrap the Rust static archive in framework bundles before calling xcodebuild, because the SDK needs PrivacyInfo.xcprivacy to live in each framework slice. The docs now call out why the tempting -library -headers path is not enough unless another App Store-visible SDK target carries the privacy manifest.

This also hardens the release path:

• tracks the generated Swift UniFFI source needed by tagged SwiftPM consumers
• declares required-reason API categories for native file metadata, disk-capacity, and elapsed-time APIs used by the embedded runtime
• adds verifiers for the privacy manifest and zipped MeshLLMFFI.xcframework.zip artifact
• verifies the macOS framework slice is versioned before accepting the release artifact
• verifies the tagged Package.swift already points at the exact release URL and checksum for the built artifact
• runs a temporary SwiftPM consumer from the zipped artifact and calls generateOwnerKeypairHex() through the packaged UniFFI binary target
• runs the zipped-artifact consumer smoke from the Swift SDK CI smoke script and release preparation script
• adds a release workflow job that builds, verifies, and uploads MeshLLMFFI.xcframework.zip
• adds stream cancellation tests for break/cancel, completion-before-activation, and late completion after cancellation

Validation

• bash -n sdk/swift/scripts/build-xcframework.sh
• bash -n sdk/swift/scripts/build-host-macos-xcframework.sh
• bash -n scripts/verify-swift-privacy-manifest.sh
• bash -n scripts/verify-swift-release-artifact.sh
• bash -n scripts/verify-swift-package-manifest.sh
• bash -n scripts/ci-swift-sdk-smoke.sh
• bash -n scripts/prepare-swift-package-release.sh
• plutil -lint sdk/swift/PrivacyInfo.xcprivacy
• scripts/check-sdk-contract.sh
• ./sdk/swift/scripts/build-host-macos-xcframework.sh
• scripts/verify-swift-privacy-manifest.sh sdk/swift/PrivacyInfo.xcprivacy sdk/swift/Generated/MeshLLMFFI.xcframework
• zipped-artifact smoke via scripts/verify-swift-release-artifact.sh <tmp>/MeshLLMFFI.xcframework.zip; this verified the versioned macOS framework slice and ran a SwiftPM consumer that calls UniFFI
• scripts/verify-swift-package-manifest.sh v0.0.0-test <tmp>/MeshLLMFFI.xcframework.zip <tmp>/Package.swift
• swift test
• workflow YAML parsed with Ruby/Psych
• cargo run -p xtask -- repo-consistency release-targets
• cargo run -p xtask -- repo-consistency ci-crate-lists

Notes

The fully native long-term shape is an Xcode-built static framework target archived per platform and then assembled into an XCFramework. This PR is a narrower step: remove hand-built XCFramework metadata now while preserving the privacy manifest behavior from #665 and adding checks that catch SwiftPM release packaging regressions.

A release tag must already contain the prepared SwiftPM URL/checksum in Package.swift; release CI now verifies that instead of mutating the manifest after the tag has been checked out.

[i386]

Addressed the review feedback in dd34aaa:\n\n- Documented that this PR depends on #665's inner framework layout fix: xcodebuild wraps input frameworks and does not repair a flat macOS bundle.\n- Added release-artifact verification that every macOS slice has the versioned Versions/A layout and canonical symlinks.\n- Strengthened the zipped SwiftPM consumer smoke to call generateOwnerKeypairHex() through the packaged UniFFI binary target, not just compile wrapper code.\n- Added a release workflow job that runs the Swift package artifact preparation path and uploads MeshLLMFFI.xcframework.zip before publication.