fix(protocol): complete motion graphics identifier rename

[ZhangchengCharles]

What changed

• rename the remaining graphic_motion cache, helper, and test identifiers to motion_graphics
• update all affected call sites without changing behavior

Why it changed

• the post-merge rigor review found remaining underscore/case variants that violated the repository-wide terminology migration

How it was checked

• 20 focused video-understand protocol/render tests passed
• Python syntax compilation passed with an isolated bytecode cache
• tracked old-name content and path scans returned zero
• git diff --check passed