test: raise coverage for src/arx/codegen.py (link=False, link_mode branches)

[omsherikar]

Summary

src/arx/codegen.ArxBuilder.build has uncovered branches for object-only output and explicit pie / no-pie link modes (~86% file coverage).

Gaps (from pytest --cov=arx --cov-report=term-missing)

• link=False: write object bytes directly to output_file and return (~90–92).
• Invalid link_mode defensive ValueError on builder (~95) — if reachable from public API, assert with a test.
• link_mode == \"pie\" / \"no-pie\": clang argument branches when linking (~103, ~105).

Suggested approach

• Add or extend tests in tests/test_codegen_ast_output.py / tests/test_codegen_file_object.py per existing CI patterns (some paths require clang).

Evidence

Recent local run: codegen.py ~42 statements, ~6 missed (~86% cover).