feat(build): add macOS-hosted Linux cross devshells

[elezar]

Summary

Add Darwin-hosted Nix cross shells for building ARM64 Linux musl CLI and glibc 2.28 gateway artifacts with plain cargo build. The generated binaries were smoke-tested in an ARM64 Ubuntu guest VM.

Related Issue

Related to #3218. This PR was directly authorized; the issue is not marked state:accepted and is not closed automatically.

Changes

• Added host-qualified crossShells.aarch64-darwin.aarch64-linux.{musl,gnu} outputs.
• Added a shared Cargo/Zig cross-shell helper that routes build commands through cargo-zigbuild.
• Added static ARM64 target Z3 for the default-feature GNU gateway shell.
• Documented the cross-shell interface and guest-artifact behavior.

Testing

• nix flake check --no-build
• mise run pre-commit passes (unavailable: mise is not installed on this host)
• Unit tests added/updated (Nix development-shell change)
• ARM64 musl CLI build through crossShells.aarch64-darwin.aarch64-linux.musl
• ARM64 GNU gateway build through crossShells.aarch64-darwin.aarch64-linux.gnu
• ARM64 Ubuntu guest-VM smoke test: both binaries ran with --version
• E2E tests added/updated (not applicable; no services were started)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)