chore: repo hygiene (.gitignore, requirements, README)

[PSamuele]

What

• Add .gitignore (Python bytecode, virtual environments, XFOIL executables)
• Stop tracking 17 .pyc files and the 2 xfoil.exe binaries
• Add requirements.txt (numpy>=2.4.4, scipy>=1.17.1, matplotlib>=3.10.8)
• Rewrite the root README (install, usage, outputs, known issues, limitations)

Why

• .pyc files are machine-generated and change on every run
• XFOIL is GPL-licensed: redistributing the binary requires shipping the license text and a source reference. On Windows the setup already downloads it automatically.

After merging (action needed on your machine)

1. The repo moved, update your remote:
   git remote set-url origin https://github.com/LS-aeroteam/Naca-Optimizer.git
2. git pull will delete xfoil_viscous_optimizer/xfoil.exe from your folder. The next run of run.py downloads it again (Windows, internet needed).
3. _Original_projects/Original_Python_Project/ has no auto-download: copy xfoil.exe there by hand if you want to run the legacy script.

Not changed

No code changes. Sub-folder READMEs are untouched (planned for a later PR).