Fix RPM digest compatibility for Fedora 43

[greymd]

Summary

This PR updates the RPM packaging pipeline to avoid does not verify: no digest failures on Fedora 43 and adds a PR-time verification workflow for RPM packages.

Changes

• switch RPM builder images from CentOS 7 to AlmaLinux 9
• set RPM file digest macros to SHA-512 in the generated spec file
• verify built RPMs on Fedora 43 in the release workflow
• add a dedicated PR workflow to build and verify RPM packages before merge
• fix aarch64 RPM builder image selection by using AlmaLinux multi-arch images with --platform linux/arm64

Why

Issue #74 reports that the current RPM package fails to install on Fedora 43 with:

package ... does not verify: no digest

The current RPMs are built with an outdated CentOS 7-based environment. This PR modernizes the RPM build environment and explicitly configures stronger file digest settings so we can validate compatibility before release.

Closes #74

Validation

• release workflow now verifies RPM installation on Fedora 43
• PR workflow builds RPMs for:
  • x86_64-unknown-linux-musl
  • aarch64-unknown-linux-musl
• PR workflow runs:
  • dnf install --nogpgcheck <rpm>
  • rpm -Kv <rpm>

Notes

• commits were created without GPG signing in the local environment because local git commit signing failed