feat(aero): Mach from stagnation pressure ratio (pitot-static)

[nochallenge]

What

Adds report::mach_from_stagnation_pressure_ratio(p0_over_p, gamma) — the Mach number recovered from a measured total-to-static pressure ratio:

// M = sqrt(2*((p0/p)^((gamma-1)/gamma) - 1)/(gamma - 1))
pub fn mach_from_stagnation_pressure_ratio(p0_over_p: f64, gamma: f64) -> f64

Why

report.rs has the #403 temperature-ratio inverse but not the pressure-ratio one. mach_from_stagnation_pressure_ratio inverts isentropic_stagnation_pressure_ratio — the subsonic pitot-static (Rayleigh-pitot) airspeed reduction, the most common compressible airspeed measurement (Mach from a measured total-to-static pressure ratio). Mirrors the #403 sentinel style: returns the at-rest 0.0 (inverse of the ratio's 1.0 no-rise identity) for non-physical input (non-finite, p0/p < 1, or gamma <= 1).

Test

mach_from_stagnation_pressure_ratio_inverts_the_ratio:

• (a) round-trip threading isentropic_stagnation_pressure_ratio (both directions);
• (b) worked — at M=1, gamma=1.4 the critical ratio p0/p = 1.2^3.5 -> M=1;
• (c) consistency with the temperature-ratio inverse (feat(astro): apsis radii from orbital elements #403) — for the same Mach, the pressure-ratio and temperature-ratio reductions return the same Mach;
• (d) at rest — p0/p = 1 -> M = 0;
• (e) guards — 0-sentinel for p0/p < 1, gamma <= 1, non-finite.

Free fn in report.rs (not re-exported), no lib.rs change. valenx-aero 213 lib tests (was 212); cargo clippy -p valenx-aero --all-targets -- -D warnings clean. Research-grade compressible-flow data-reduction primitive.