Standardize result-attribute naming on MgVideo (e.g. ssm_fig → ssm_figure)

[alexarje]

Background

Methods stash their results on the parent object under inconsistent attribute names:

• _video: motion_video, history_video, eulerian_video, motionvectors_video, blur_faces_video, pose_video, warp_video
• _image: heatmap_image, mhi_image, stroboscope_image, blend_image
• oddballs: ssm_fig (vs _figure used elsewhere), subtract (no suffix), motion_plot, ssm_combined, pixelarray, motiongram_x/y, videogram_x/y

Task

Adopt a single convention (e.g. <name>_video / _image / _figure) and rename the outliers.

Caveats (why this is deferred / 2.0-scope)

• Renaming these attributes is breaking: show(key=...) resolves some of them by attribute name, and user code/tests may reference them.
• Best done with deprecation aliases (keep the old attribute pointing at the new one for a release) and targeted for a major version (2.0).