Tracks the feature requested in discussion #43 (beauty-ad local retouch that follows the product on the face).

Gap: mask writes static mask geometry and keyframe animates segment transform (position/scale/rotation/alpha/volume), but mask geometry itself is not keyframeable today.

Scope:

• Add mask position / width / height / rotation / feather as keyframeable properties (via keyframe --batch).
• Helper to duplicate a source clip onto an upper PIP layer and attach the local mask there, leaving the lower track untouched.
• Validation report: overlay count, mask count, keyframe count, missing-media count.

Notes: direct draft editing only (encrypted JianYing 6.0+ out of scope, already flagged by decrypt/version). Requester offered a sample draft + source video + screenshots to build against.