ci(release): de-risk macos-26 + Xcode 26.4 with an RC tag before next release

[Mx-Iris]

Context

PR #65 bumped `.github/workflows/release.yml` to `macos-26` + Xcode `26.4`, but `release.yml` only runs on tag push (`on.push.tags`), so the new env has not actually been exercised end-to-end yet. The release workflow does:

1. `swift build -c release --arch x86_64 --product swift-section`
2. `swift build -c release --arch arm64 --product swift-section`
3. `lipo -create x86_64 arm64 -output Products/swift-section`
4. `codesign --force --sign - ./Products/swift-section`

x86_64 cross-compilation on the Apple Silicon `macos-26` runner with the macOS 26 SDK is the unverified piece.

Suggested action

Before cutting the next real release, push an RC tag (e.g. `vX.Y.Z-rc1`) and let `release.yml` run end-to-end on it. Confirm:

• Both `swift build` invocations succeed.
• `lipo` produces a valid universal binary.
• Ad-hoc `codesign` succeeds and the resulting binary runs (`file Products/swift-section` → `Mach-O universal binary`).

If anything fails, fix it on a branch before tagging the real release.

Priority

Medium — required before the next release tag.

Source

Code review on PR #65 (post-merge follow-up).