plot_section_polars covers only POLAR_VECTORS: plot cl, cd, cm against alpha per panel for every aero model #331

The "panel polar investigation" planned in #113 is only partly there: plot_section_polars plots one of :cl, :cd, :cm per call and only for sections with POLAR_VECTORS data (ext/VortexStepMethodMakieExt.jl:1508; other sections are skipped, and it errors when none are left). plot_polar_data draws POLAR_MATRICES as 3D (alpha, delta) surfaces for panel 1 only. Nothing plots what a panel actually uses for POLY (Breukels), INVISCID or POLAR_MATRICES at a given flap deflection.

What @jellepoland described in #113: 2D plots of cl, cd and cm against alpha, to check whether the underlying method — CSV polar, polynomial, inviscid or anything else — is smooth and has the expected sign. Python had it as plot_panel_coefficients(body_aero, panel_index, alpha_range=[-20, 30]) (awegroup/Vortex-Step-Method@48d4e93, "#92 panel polar investigation"); it is commented out on Python main today (src/VSM/plotting.py:700), so there is nothing current to port.

Proposal: one figure per call with cl, cd and cm against alpha, evaluated through the panel's own coefficient functions (calculate_cl, calculate_cd, calculate_cm in src/panel.jl) so every aero model goes through the same path, for a chosen panel or list of panels, with the flap deflection as a keyword for POLAR_MATRICES. This extends plot_section_polars rather than adding a second function.

Found while surveying the Python package for #113.