fix(hardrt): support historical 0.5.1 finalization

[Inczert]

Summary

Allow the standardized release finalizer to complete the already-published HardRT 0.5.1 release without rewriting its immutable tag or historical software artifacts.

• canonical future contract remains native Linux amd64 + arm64 POSIX packages, Cortex-M, and SHA256SUMS
• exact version 0.5.1 may instead use its already-published historical generic POSIX + Cortex-M + bundle + SHA256SUMS set
• the historical exception cannot apply to any later version
• whichever accepted software set is used is downloaded and verified against its published checksum before physical evidence is attached
• documentation records why the exception exists and explicitly forbids carrying the generic POSIX/bundle layout forward

This enables scripts/finalize_release.sh 0.5.1 ... --cleanup-branches to attach the retained STM32 qualification .tar.xz evidence and finish release cleanup with the same standardized finalizer used by future releases.