Add Python-side Stokes PC fallback recorder

[ss2098]

This PR fixes an AttributeError in the Python-side Stokes preconditioner fallback check.

_check_velocity_preconditioner() records when the velocity block falls back from the intended multigrid preconditioner, but the Python-side Stokes object did not have _record_pc_fallback(). A normal Stokes solve could therefore fail with:

AttributeError: 'SNES_Stokes' object has no attribute '_record_pc_fallback'

This adds a small Python-side fallback recorder and pc_fallbacks property so the fallback is recorded and the warning path can complete.

This does not change solver setup or preconditioner selection.

Validation:

• python -m py_compile src/underworld3/systems/solvers.py
• pixi run python docs/examples/Tutorial_Thermal_Convection_Units.py

The units tutorial now completes successfully. The remaining GAMG message is the intended runtime warning, not a crash.