Cleanup: Remove temporary debug and demo scripts from root

Deleted 79 temporary Python scripts from root directory:
- 69 test_*.py files (untracked, debug scripts from units development)
- 10 tracked demo/debug files:
  - debug_parameter_substitution.py
  - demo_jit_debugging.py
  - demo_nondimensional_viewer.py
  - demo_practical_use.py
  - demo_rounding_limitations.py
  - demo_simple_nondim.py
  - demo_unit_conversion.py
  - demo_universal_nondim.py
  - mesh_units_examples.py
  - mesh_units_simple.py

Kept:
- setup.py (required for building Cython extensions)

Rationale:
All deleted scripts were temporary investigation/demonstration code
created during units system development. Functionality is covered by
the official test suite in tests/.

Note: The .gitignore already had /test_*.py pattern, so 69 test files
were untracked. This commit removes the 9 tracked demo/debug scripts.

Author: lmoresi