Core: batch process all bones at each frame

[Lauloque]

Currently, each bone is processed one at a time, selecting/deselecting each bone individually:

# Current: O(bones * frames) selection operations
for bone in bones:
    setup_bone_for_conversion()  # select/deselect
    for frame in frames:
        convert_frame()

This current implementation is a limitation from using Copy Rotation Transform addon. Once #48 is completed, it might allow a better approach:

# Process all bones per frame: O(frames) selection operations  
for frame in frames:
    contert_all_bones()