Re-enable Linux arm64 release artifacts

[donbeave]

Summary

• add native aarch64-linux to the release matrix using ubuntu-24.04-arm
• wire npm/cli-linux-arm64 through validation, artifacts, publishing, optional dependencies, and the npm launcher
• add package.metadata.binstall so existing release archives are discoverable by cargo binstall shellfirm
• document the cargo binstall shellfirm prebuilt install path

Closes #179

Verification

• cargo package --locked --allow-dirty -p shellfirm
• cargo binstall shellfirm --manifest-path shellfirm/Cargo.toml --target aarch64-apple-darwin --dry-run --no-confirm --log-level info downloaded the GitHub release binary
• cargo binstall shellfirm --manifest-path shellfirm/Cargo.toml --target x86_64-apple-darwin --dry-run --no-confirm --log-level info downloaded the GitHub release binary
• cargo binstall shellfirm --manifest-path shellfirm/Cargo.toml --target x86_64-unknown-linux-gnu --dry-run --no-confirm --log-level info downloaded the GitHub release binary
• cargo binstall shellfirm --manifest-path shellfirm/Cargo.toml --target aarch64-unknown-linux-gnu --dry-run --no-confirm --log-level info falls back to source for v0.3.10 as expected because that release does not yet include shellfirm-v0.3.10-aarch64-linux.tar.xz
• native Linux arm64 Docker proof: cargo test --release --locked --target aarch64-unknown-linux-gnu, cargo build --release --locked --target aarch64-unknown-linux-gnu, strip, archive as shellfirm-v0.3.10-aarch64-linux.tar.xz, shellfirm --version, and file all passed; produced an ELF 64-bit LSB pie executable, ARM aarch64
• Linux x64 Docker proof: cargo build --release --locked --target x86_64-unknown-linux-gnu, strip, shellfirm --version, and file passed; produced an ELF 64-bit LSB pie executable, x86-64
• macOS local proof: cargo build --release --locked --target aarch64-apple-darwin and cargo build --release --locked --target x86_64-apple-darwin passed; file reports Mach-O 64-bit executable arm64 and Mach-O 64-bit executable x86_64
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/release.yml"); puts "release workflow yaml ok"'