Update adapt cost microbenchmark for current APIs

[ss2098]

This PR updates scripts/adapt_cost_microbench.py so the microbenchmark runs with the current Underworld3 APIs.

Changes include:

• convert AdvDiffusionSLCN estimate_dt() output to a scalar before passing it to solve()
• replace internal mesh._deform_mesh() calls with the public mesh.deform() API
• replace the retired smooth_mesh_interior(..., method="anisotropic") path with uw.meshing.node_redistribution()
• add fail-fast SNES convergence checks so the benchmark does not continue after a failed solve
• simplify the Stokes boundary condition so the microbenchmark runs stably as a cost diagnostic

This does not change solver internals.

Validation:

• pixi run python scripts/adapt_cost_microbench.py

Example local output:

• plain (adv+stokes) step: 1.217 s
• one pristine adaptation total: 3.586 s
• adaptation / step ratio: 2.9x

[lmoresi]

Adversarial review

The API modernizations are correct (we live-probed node_redistribution, metric_density_from_gradient(amp=, name=), Mesh.deform, .snes — all present), but two substitutions change what the benchmark measures:

1. MAJOR — the upper boundary changed from penalty free-slip to no-slip (add_essential_bc((0,0))). That silently changes the convection physics while the docstring still claims "same setup as the saturation runner", so the numbers are no longer comparable to the recorded baselines this script exists to track. add_natural_bc still exists; the sanctioned modernization is add_rotated_freeslip_bc(0, "Upper").
2. MAJOR — the timed Stokes solves flipped zero_init_guess=False → True: the benchmark now times cold-start solves, inflating t_step and deflating the adaptation/step cost ratio.
3. MINOR — _deform_mesh → deform: deform() runs remesh_with_field_transfer itself before the script overwrites T, so the remap timings now double-count transfer work.

Requested changes: restore free-slip via add_rotated_freeslip_bc(0, "Upper"), restore zero_init_guess=False in the timed loops (or explicitly re-baseline and relabel), and note the deform double-transfer in the timing comment. With those, this is mergeable — the rest of the update checked out clean.