Record alpha.12 signed-install checkpoint

[R055LE]

Summary

• record the permanent-key alpha.12 artifact identity and successful installation
• keep alpha.11 as the accepted tester build until the functional checklist passes
• record the temporary launcher icon as good enough for this checkpoint with exact sizing still pending
• add a dated investigation and signing-history entry

Verification

• checksum matches the signed APK
• package is io.github.r055le.threadline, version 0.1.0-alpha.12 (10012)
• APK signature schemes v2 and v3 verify against the established release certificate
• 16 KiB page alignment passes
• embedded version-control record identifies exact merged-main commit ad120ce4a9527960dbf85cac01139266b358e76e

Documentation only. The owner-device functional checklist remains pending.