Fix Liquid Glass release toolchain gating

[yvette-carlisle]

Summary

• pin Swift and release GitHub Actions jobs to macos-26 instead of floating macos-latest
• fail release staging when Apple Swift is older than 6.2 and include the Swift toolchain in the stage fingerprint
• clarify Liquid Glass Settings fallback copy and align README/spec/release runbook wording
• remove the old GitHub Actions-only swift-format workaround and route Swift format lint through cargo make lint-swift-format

Root cause

The v0.1.2 GitHub release was built on macos-latest, which resolved to macos-15-arm64 with SDK 15.5. The current Liquid Glass path is guarded by #if compiler(>=6.2), so that release artifact compiled out the Liquid Glass code even when run on macOS 26.4.

Validation

• actionlint .github/workflows/language.yml .github/workflows/release.yml
• cargo make lint-swift-format
• cargo make lint-swift
• cargo make test-macos-native-host
• cargo make test-macos-native-host-stage
• release-style RSNAP_NATIVE_HOST_RUST_PROFILE=final-release RSNAP_NATIVE_HOST_SWIFT_CONFIGURATION=release ./scripts/build_and_run.sh stage
• fake Swift 6.1 release-stage fail-closed probe
• git diff --check

Notes

Existing v0.1.2 release artifacts still lack Liquid Glass. A new release is required for users to receive a bundle compiled with the macOS 26 SDK.