fix(release): compare signed appcast enclosure

[hcaiano]

Summary

• compare the full generated and committed Sparkle enclosure lines
• treat URL-only matches as changed when signature or length differs
• keep true no-op reruns free of appcast commits

Test plan

• swift build
• swift run cycler-tests (ok — 106 checks passed)
• actionlint .github/workflows/release.yml
• bash -n Scripts/sparkle-appcast.sh
• shell assertion: identical enclosure is unchanged and modified length is changed
• git diff --check

Runtime evidence

• successful run 29111015018 rebuilt and uploaded a 4177304-byte DMG while the existing appcast still signed 4177027 bytes; URL-only comparison skipped the required appcast update