fix: terminal messages and robustness

[PSamuele]

What

• "Found XFOIL executable" is printed once (start-up check), no longer inside the optimization table.
• XFOIL graphics are disabled in the input script (PLOP > G): no plot windows, no crash on systems without a display.
• If no airfoil is analysed successfully, the run stops with a clear message instead of reporting "OPTIMIZATION COMPLETE" with a meaningless airfoil.
• XFOIL optimizer: final Cl and Cd come from a new XFOIL run on the optimized airfoil (the last history row can be a finite-difference point). A warning is printed if XFOIL does not converge or stops at a different alpha.
• Validation: one line per angle (the panel method runs quietly).
• Missing Python libraries: the script asks before running pip. If you answer no, it prints the pip install -r requirements.txt command.
• Results folder name uses the rounded Reynolds number, the same one printed on screen (3424658 instead of 3424657).
• The XFOIL polar file is passed by name, since XFOIL runs inside the temporary folder.

Verified

• Baseline 24/24.
• Windows: all three scripts run; "Found XFOIL" appears once; validation prints one line per angle; the XFOIL optimizer gives the same numbers as before.
• Linux sandbox: XFOIL runs without a display with graphics disabled by the project; with XFOIL forced to fail, the run stops with the new message; with a missing library and answer "no", the install command is printed.

Not changed

Optimization algorithms and terminal terms ("iterations", "Genetic Algorithm").

Depends on

#6 (base branch: fix/output-files).