[Testing]: Differential tests — _grep_python vs system grep #63

Problem

GCode ships a pure-Python grep fallback (_grep_python) for Windows, but it
is only compared against expected strings, not against a real grep on the
same corpus. Discrepancies in matching (regex flavor, --glob, case
insensitivity, binary detection) go unnoticed.

Proposed approach

• Build a fixture corpus (text, nested dirs, binary file, files matching/not
  matching a glob).
• On platforms with grep, assert _grep_python(...) and grep -r ...
  produce equivalent matches for: plain regex, -i, glob filter, no-match.
• Keep tests skippable when grep isn't available.

Where to look

• gcode/tools.py:242-285 (_grep_python), :192-229 (grep tool)
• Existing tests/test_tools.py grep tests.

Acceptance criteria

• New tests/test_grep_differential.py passes on macOS/Linux, skips cleanly on Windows.

Difficulty

Low — good first issue.