Fix VFP1D anisotropic collision matrix

[joglekara]

Summary

• correct the FLM b2 coefficient sign used by the f10 collision operator
• pack the row-centered lower and upper tridiagonal entries correctly and enforce spherical-harmonic parity at v=0
• scale the electron-electron angular collision term by the requested harmonic index
• add coefficient, matrix-assembly, boundary-parity, and drifting-Maxwellian refinement regressions

Validation

• pre-commit run --files adept/vfp1d/fokker_planck.py tests/test_vfp1d/test_flm_collisions.py
• targeted collision/harmonics tests: 34 passed
• full VFP1D suite with local MLflow tracking: 51 passed, 24 skipped, 11 deselected