refactor: shared naca_core package

[PSamuele]

What

• New naca_core/ package with the modules that were duplicated in inhouse_core/ and xfoil_core/: airfoil, panel_method, plotting, pre_run_checks, xfoil.
• Optimizers moved next to their run.py: inhouse_potential_optimizer/inhouse_optimizer.py and xfoil_viscous_optimizer/xfoil_optimizer.py.
• One pre_run_checks with perform_all_checks(require_xfoil=...). XFOIL is still searched for (and downloaded to) xfoil_viscous_optimizer/.
• numpy/scipy/matplotlib are imported after the environment check. Before, a missing library crashed the script before the check could run.
• The validation now uses the same panel method as the in-house optimizer (before, it used the copy in xfoil_core).
• README: new project structure and a "Checking your changes" section.

Commits

1. Moves only: git shows 100% renames, so file history is preserved.
2. Import updates.

Verified

• python tests/baseline_check.py: 24/24 match.
• All three scripts run end to end on Windows with default inputs.
• In a Linux sandbox, the terminal output of the XFOIL optimizer and of the validation is identical before and after.

Not changed

No numerical or terminal-output changes. Known bugs (.dat line breaks, SVG format, XFOIL messages inside the table, ...) are fixed in the next PR.

After merging

Your local inhouse_core/ and xfoil_core/ folders may remain with only __pycache__ inside: they are ignored by git and can be deleted.

Depends on

#4 (base branch: test/numerical-baseline).