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