Recession τ and b priors from hillslope geometry + POLARIS

[awickert]

Forward map from hillslope geometry (L, β, H; #29) and POLARIS soil properties (K, f; #30) to recession timescale and exponent priors, via hillslope Boussinesq theory:

• advection τ ≈ L·f / (K·sinβ)
• diffusion τ ≈ L²·f / (K·H) (same D = KH/f as the matrix-identifiability work)
• power-law exponent b from the nonlinear Boussinesq solution

Complements the inverse Brutsaert–Nieber recession analysis with a geometry-based forward prior. Foundation for v4.0 field-data priors.

Depends on #29 + #30. Context: the recession-from-hillslope-geometry notes.

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