Add script to build and publish the Darwin Swift SDK to GHCR

[JayShortway]

Checklist

• If applicable, unit tests
• If applicable, create follow-up issues for purchases-android and hybrids

Motivation

To cross-compile RevenueCat/RevenueCatUI for Apple platforms from Linux (e.g. Cursor cloud agents) via xtool, we need a Darwin Swift SDK extracted from Xcode. Since Xcode can't be redistributed and downloading it requires interactive Apple auth, this provides a maintainer-run script to produce that SDK once and publish it privately for reuse.

Description

Adds scripts/build-darwin-sdk.sh, which:

• resolves an Xcode source (an already-installed Xcode.app, a provided .xip, or a version downloaded via xcodes using a pre-generated FASTLANE_SESSION to avoid interactive 2FA),
• runs xtool sdk build to extract a redistributable Darwin Swift SDK .artifactbundle (target host arch configurable; defaults to x86_64 to match cloud agents),
• packages it as a gzipped tarball and pushes it to GHCR as an OCI artifact via oras, with version/arch/checksum annotations.

The script header documents how to consume the artifact in a .cursor environment Dockerfile (public base image + a RUN --mount=type=secret step + swift sdk install), which is the Cursor-supported pattern — a private FROM base image is intentionally avoided since Cursor's builder has no documented way to authenticate a private base-image pull.

Validated with shellcheck (clean) and by exercising the argument-validation and dependency-check paths. The end-to-end build/push path requires Xcode + oras and was not run in CI.

  

[RevenueCat-Danger-Bot]

	1 Error
🚫	Label the PR using one of the change type labels. If you are not sure which label to use, choose pr:other.

Label	Description
pr:feat	A new feature. Use along with pr:breaking to force a major release.
pr:fix	A bug fix. Use along with pr:force_minor to force a minor release.
pr:other	Other changes. Catch-all for anything that doesn't fit the above categories. Releases that only contain this label will not be released. Use along with pr:force_patch, or pr:force_minor to force a patch or minor release.
pr:RevenueCatUI	Use along any other tag to mark a PR that only contains RevenueCatUI changes
pr:next_release	Preparing a new release
pr:dependencies	Updating a dependency
pr:phc_dependencies	Updating purchases-hybrid-common dependency
pr:changelog_ignore	The PR will not be included in the changelog. This label doesn't determine the type of bump of the version and must be combined with pr:feat, pr:fix or pr:other.

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