docs: reference section and subfolder READMEs

[PSamuele]

What

• Main README: new "Reference" section with every symbol, terminal message, table column, score formula, validation mark, value (with where it is used and why) and condition. Values that come from the original code and were never tuned are marked as such.
• Subfolder READMEs rewritten to match the code. Removed statements that were not true: "No External Dependencies", "sub-millimeter refinement", automatic install "without prompting", "legacy code", exported "XFOIL polars".
• Each subfolder README links to the Reference section.
• Roadmap: "One shared core package" removed (done in refactor: shared naca_core package #5).

All numbers quoted come from the runs documented in the previous PRs.

Depends on

#12 (base branch: feat/validation-fallback).