Record Android alpha21 release

[RioPlay]

Summary

• record the exact alpha21 build, signing, publication, tag, checksum, version, and certificate evidence
• mark alpha21 as the current signed Android preview
• close the bounded release plan while preserving physical-device and final A-M limits
• correct stale current-release and workspace routing references

Verification

• public APK SHA-256 matches SHA256SUMS.txt and GitHub asset digest
• version.json reports org.utterleaf.voice, code 21, 0.1.0-alpha21
• public certificate matches the established release fingerprint
• tag resolves to exact green main revision 394fdff
• independent release-record review: no remaining findings