feat(fem): polar second moment of area of a circular shaft

[nochallenge]

What

Adds beam::circular_polar_second_moment_of_area(diameter) — the polar second moment of a solid round shaft about its longitudinal axis:

// J = pi * d^4 / 32   (m^4)
pub fn circular_polar_second_moment_of_area(diameter: f64) -> f64

Why

The torsion constant of a round bar — the J that feeds polar_section_modulus (Z_p = J/(d/2)) and torsional_rigidity (GJ), neither of which previously had a way to compute J for a circle (both take it as a parameter). By the perpendicular-axis theorem it is exactly twice the bending circular_second_moment_of_area (J = Ix + Iy = 2*I, since Ix = Iy for a circle). Guard mirrors the bending companion.

Test

circular_polar_second_moment_of_area_is_pi_d4_over_32:

• (a) worked — d=0.1 -> J = pi*0.1^4/32 ~= 9.8175e-6 m^4;
• (b) threads circular_second_moment_of_area (feat(cfd): vorticity standard deviation (variance identity) #419) (non-tautological, perpendicular-axis theorem) — J = 2*I;
• (c) threads polar_section_modulus — Z_p = J/(d/2) = pi*d^3/16;
• (d) threads torsional_rigidity (feat(aero): Mach from stagnation temperature ratio (isentropic inverse) #410) — GJ of a steel shaft;
• (e) scaling — quartic in diameter;
• (f) guards — 0-sentinel for non-positive d and non-finite input.

Re-exported from lib.rs alphabetically in the beam block. valenx-fem 261 lib tests (was 260); cargo clippy -p valenx-fem --all-targets -- -D warnings clean. Research-grade torsion section-property primitive.