test: cover ArxBuilder link and link_mode paths

[omsherikar]

Exercise link=False object output, invalid link_mode, and pie/no-pie clang flags (closes codegen coverage gaps).

solves #42

[Copilot]

Pull request overview

This PR adds targeted tests to close code coverage gaps in src/arx/codegen.py around object-only output (link=False), invalid link_mode handling, and explicit pie / no-pie linker-flag selection.

Changes:

• Add a minimal module parsing helper to reduce repetition in codegen tests.
• Add a test asserting link=False writes the emitted object bytes directly to output_file.
• Add tests for invalid link_mode and for forwarding -pie / -no-pie to the clang invocation.

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[Copilot]

test_build_passes_explicit_link_flags monkeypatches arx.codegen.xh.clang, so it no longer needs a real clang binary on PATH. The current @skipif(not HAS_CLANG, ...) will unnecessarily skip this coverage in environments without clang and the skip reason is misleading; consider removing the skip or changing the condition/reason to reflect the actual prerequisite (e.g., only skip when LLVM object emission isn’t available).

Suggested change

@pytest.mark.skipif(not HAS_CLANG, reason="clang is required for object build")