Add multi-architecture Linux release packaging workflow

[RchGrav]

Motivation

• Provide multi-architecture release artifacts for both GNU/glibc and musl targets so prebuilt binaries can be published for a wide range of Linux platforms.
• Use CI-only cross-compilation approaches (dockcross for GNU cross builds and Zig for musl cross-compilation) while keeping the repo's native make build as authoritative.

Description

• Add a GitHub Actions workflow .github/workflows/release.yml that triggers on v* tags and also runs on PRs/pushes to main, with jobs: test (ubuntu amd64), build-gnu (matrix for linux-amd64-gnu, linux-arm64-gnu, linux-armhf-gnu using native or dockcross), build-musl (matrix for multiple musl targets using pinned Zig), and a tag-only release job that gathers artifacts and publishes tarballs + SHA256SUMS to the GitHub Release.
• Add .github/scripts/package_tarball.sh which produces sigmund-<version>-<target>.tar.gz archives containing a stable sigmund executable (executable bit preserved) and any LICENSE*/README* files when present.
• Musl builds use a pinned Zig release (0.13.0) and attempt -static first, falling back to a dynamic musl build on failure so the workflow does not fail entirely for a single non-staticable target.
• Artifact naming and contents follow the requested format (sigmund-<version>-<target>.tar.gz, version derived from tag vX.Y.Z as X.Y.Z), and a SHA256SUMS file is generated in the release job.
• No program source files or tests were modified.

Testing

• Ran the shell lint for the packaging script with bash -n .github/scripts/package_tarball.sh which completed successfully.
• Built and ran the repository test suite with make clean && make && make test on the local ubuntu environment and all tests passed.
• Validated workflow YAML syntax with ruby -e "require 'yaml'; YAML.load_file('.github/workflows/release.yml')" which loaded successfully.

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Codex Task

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