FreeSurface: composition transport is level-set-only; can't drive temperature convection without private access

[lmoresi]

Summary

uw.systems.FreeSurface accepts a composition field and advects it with the material-consistent surface velocity (the correct way to keep a free surface a material boundary). The constructor docstring lists temperature as an intended use ("a mesh level set holding a signed distance, or temperature"). But _build_composition_transport hardcodes level-set defaults that are wrong for a temperature/convection field:

• diffusivity = 1.0e-7 (a level set carries no real diffusion; temperature needs the thermal κ),
• bcs = [] (temperature needs Dirichlet BCs at the surface/base),
• monotone_mode = None (a bounded [0,1] temperature typically wants monotone limiting).

Impact

Building a free-surface convection driver on the manager currently requires reaching into the private transport solver after construction:

fs = uw.systems.FreeSurface(stokes, "Upper", ..., composition=T)
fs._comp_adv.constitutive_model.Parameters.diffusivity = kappa   # private
fs._comp_adv.add_dirichlet_bc(1.0, "Lower")                      # private
fs._comp_adv.add_dirichlet_bc(0.0, "Upper")                      # private

This works (a clean annulus FS-convection run is up and stable on it) but is fragile — it depends on the private _comp_adv name and on BCs being addable post-construction.

Suggested resolution (either)

1. Parameterize the composition transport: composition_diffusivity, composition_bcs, composition_monotone on the constructor; or
2. Expose the material-consistent advection velocity publicly (e.g. fs.advection_velocity) and run the consistent solve even when composition is None, so callers can drive their own AdvDiff with proper κ/BCs/monotone.

Option 2 is more flexible (temperature convection wants its own AdvDiff anyway); option 1 is more turnkey. Worth a design decision.

Context

Found while building a fresh annulus free-surface convection driver on the hardened FreeSurface manager (feature/free-surface-tangent). Reference: docs/developer/design/FREESLIP_DYNAMIC_TOPOGRAPHY_FREESURFACE.md, skill free-surface-convection.

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