REFAC: demote Python-era C dual-call APIs to file-local helpers

[Dengda98]

• After REFAC: replace in-memory Python–C bindings with grt CLI workflow #287 moved Python heavy computation to the grt CLI, remove C-level compromises that existed for in-memory Python/ctypes dual calling.
• Demote strain/rotation/stress, syn, and GF-prepare kernels to static helpers; delete unused dyn_postprocess.h / static_postprocess.h.
• Simplify greenfn Ctrl by dropping unused derived fields (nf/df/winT/wI/freqs, H.nf1/nf2) and free grn->freqs in the main path.
• Update outdated comments that still referred to Python callers.