feat(aero): Mach from stagnation density ratio (completes the trio)

[nochallenge]

What

Adds report::mach_from_stagnation_density_ratio(rho0_over_rho, gamma) — the Mach number recovered from a measured total-to-static density ratio:

// M = sqrt(2*((rho0/rho)^(gamma-1) - 1)/(gamma - 1))
pub fn mach_from_stagnation_density_ratio(rho0_over_rho: f64, gamma: f64) -> f64

Why

report.rs had the temperature- (#403) and pressure-ratio (#409) Mach inverses; this is the density-ratio one, inverting isentropic_stagnation_density_ratio and completing the stagnation-ratio inverse trio. Mirrors their sentinel style: returns the at-rest 0.0 (inverse of the ratio's 1.0 no-compression identity) for non-physical input (non-finite, rho0/rho < 1, or gamma <= 1).

Test

mach_from_stagnation_density_ratio_inverts_the_ratio:

• (a) round-trip threading isentropic_stagnation_density_ratio (both directions);
• (b) worked — at M=1, gamma=1.4 the sonic ratio rho0/rho = 1.2^2.5 -> M=1;
• (c) consistency — for the same Mach, the density-, pressure-, and temperature-ratio reductions all return the same Mach (cross-validates the whole trio, non-tautological);
• (d) at rest — rho0/rho = 1 -> M = 0;
• (e) guards — 0-sentinel for rho0/rho < 1, gamma <= 1, non-finite.

Free fn in report.rs (not re-exported), no lib.rs change. valenx-aero 214 lib tests (was 213); cargo clippy -p valenx-aero --all-targets -- -D warnings clean. Research-grade compressible-flow data-reduction primitive.