Polish release workflow

[DaniusKalv]

Summary

Five workflow fixes plus the armhf build foundation.

• Asset names use + instead of ~ (GitHub release uploads rewrite ~ to .).
• Asset filename pattern: libcamera_<full-version>_<suite>_<arch>.tar.gz (Debian-style underscore-separated fields).
• Release page targets the source tag (v...) instead of the packaging tag (debian/...), so the GitHub diff view spans only the upstream changes.
• New releases ship as pre-releases with a skeleton notes template (sensor list, added features, hardware validation as fillable section anchors).
• Release title matches the source tag (e.g. v0.7.1+rpt20260429+krks1).

Plus arch input + arch-test on _build.yml as a foundation for future cross-arch builds. Investigation confirmed armhf builds natively at ~1.34x the arm64 baseline. Matrix stays at 2 entries (trixie/arm64, bookworm/arm64). armhf can fold in when a 32-bit release is needed.

Bumps debian/changelog to 0.7.1+rpt20260429+krks1-2.

Test plan

• release.sh from debian/latest produces a release at v0.7.1+rpt20260429+krks1.
• Release is marked pre-release, carries the skeleton notes, two tarballs + SHA256SUMS + SHA256SUMS.asc.
• sha256sum -c SHA256SUMS and gpg --verify SHA256SUMS.asc SHA256SUMS both pass.
• Hardware validate on Pi 4 / Pi 5.