ci: cross-platform release matrix (Linux x86_64, macOS arm64, Windows, Linux aarch64)

[jabir-dev788]

Summary

Resolves #130. Replaces the single Linux-only build job with a 4-target
matrix that builds and publishes release artifacts for all platforms
specified in the acceptance criteria.

Changes

• .github/workflows/release.yml — only file modified

Build Matrix

Runner	Rust Target	Artifact
ubuntu-latest	x86_64-unknown-linux-gnu	starforge-linux-x86_64.tar.gz
macos-latest	aarch64-apple-darwin	starforge-macos-aarch64.tar.gz
windows-latest	x86_64-pc-windows-msvc	starforge-windows-x86_64.zip
ubuntu-latest	aarch64-unknown-linux-gnu	starforge-linux-aarch64.tar.gz

Implementation Notes

• fail-fast: false so all 4 jobs run to completion independently
• aarch64 Linux uses gcc-aarch64-linux-gnu (apt) as cross-linker — no Docker required
• macOS target is aarch64-apple-darwin since macos-latest is an M1 runner
• Windows packaging uses PowerShell Compress-Archive for .zip
• Existing trigger (push: tags: v*) and permissions unchanged

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[jabir-dev788]

Hi! This PR implements the cross-platform matrix build from issue #130.
Changes are scoped to .github/workflows/release.yml only.
Could you approve the workflow run when you get a chance? Happy to
address any feedback. Thanks!