Add flow-field physics regression tests

[sergioald]

Summary

Adds focused physics/regression tests for the free-flow solver without changing solver behaviour.

Tests added

• Zero curvature gives zero migration velocity.
• Free-flow response is linear in curvature amplitude.
• NumPy serial and threaded mode accumulation agree.
• Invalid backend names are rejected.
• Optional Numba SL=0 backend agrees with NumPy when Numba is installed.
• Optional Numba SL=1 discrepancy is documented as xfail until investigated.

Validation

python -m py_compile tests/test_flowfield_physics_regression.py tests/test_flowfield_backend_guardrails.py
python -m pytest tests/test_flowfield_physics_regression.py tests/test_flowfield_backend_guardrails.py
python -m pytest