Harden test harness and prepare crash-only fuzz/corpus testing

[leo-aa88]

Summary

Improve the test harness and lay groundwork for feeding malformed or random input to the interpreter to ensure it fails cleanly.

Why

For a C interpreter, “no segfault on garbage input” is a meaningful quality bar. Before fuzzing, the harness itself should also be a bit cleaner and more robust.

Proposed changes

• Refactor test harness to avoid shell=True when possible

• Pass subprocess arguments as argv arrays

• Add a small crash-only corpus of malformed programs

• Assert that invalid input:

  • does not segfault
  • does not abort unexpectedly
  • returns controlled errors

Future extension

• Add fuzzing later, once the harness and crash expectations are clean
• Possibly maintain a corpus of pathological inputs

Acceptance criteria

• Test harness no longer relies on shell invocation unnecessarily
• Crash-only malformed input tests exist
• Interpreter fails cleanly on garbage input
• This work makes later fuzzing easier

Notes

This issue is half harness cleanup, half groundwork for future robustness testing.