Prototype Linux ARM64 zigbuild

[bkataru]

Adds a manual, non-blocking workflow for Linux ARM64 release artifact prototyping via cargo-zigbuild.

Scope:

• workflow_dispatch only.
• Linux ARM64 only.
• Does not replace release workflow native cross toolchain.
• Pins Zig 0.14.1 and cargo-zigbuild 0.22.3.
• Validates resulting artifact with file and grep "ARM aarch64".

Evidence:

• Workflow text validation passed.
• git diff --check passed.
• cargo check -p pawan --bin pawan passed locally.

Next after merge:

• Manually run workflow.
• Compare artifact behavior before any release workflow cutover.

[LuuOW]

Technical audit: Implementation verified for system consistency and engineering integrity.