fix(zero-c): fail closed for unsupported aarch64 ELF IR

Summary

Addresses #145.

This PR updates the AArch64 ELF backend so unsupported IR fails closed with a CGEN004 diagnostic instead of allowing successful emission of a misleading executable.

The change is intentionally limited to validation and diagnostic behavior. It does not add new AArch64 code generation support.

What changed

• Added AArch64 ELF validation for every IR function before successful object or executable emission.
• Preserved the existing emitted-function behavior: non-exported functions are validated but not newly emitted.
• Preserved the currently supported small literal-return subset.
• Aligned unsupported-lowering diagnostic wording with existing native backend terminology.
• Kept the existing CGEN004 diagnostic path.
• Avoided adding world.out.write lowering, Linux write syscall lowering, general call lowering, or broader backend support.

Validation

• Rebuilt the native compiler with make -C native/zero-c.
• Confirmed bash bin/zero --version succeeds.
• Confirmed zero new cli hello succeeds.
• Confirmed zero check . succeeds.
• Confirmed zero test . succeeds.
• Confirmed zero build . fails with CGEN004 for unsupported AArch64 ELF lowering.
• Confirmed zero run . fails with the same diagnostic.
• Confirmed the previous silent no-output executable behavior no longer reproduces.
• Confirmed the committed scope is limited to native/zero-c/src/emit_elf_aarch64.c.

Scope

This PR does not change:

• IR generation
• checker behavior
• CLI behavior
• x64 ELF backend behavior
• Mach-O backend behavior
• COFF backend behavior
• WASM/WASI behavior
• documentation
• package files
• lockfiles
• build scripts

Notes

This is the fail-closed compiler behavior fix. It does not make unsupported AArch64 programs run successfully. Follow-up work can add actual AArch64 lowering for specific constructs in separate PRs.