fix(studio): a move after a resize, an undo or Add keyframe lands where it is let go

[miguel-heygen]

What

Draft, stacked on #5045 (its bench cases are the red cases here); retargets to main once #5045 merges.

Studio drops or misplaces a move in three gesture sequences:

• Resize, undo, then drag on a nested element: after the undo settles, the drag does not move the box and nothing is saved (seqresizeundodrag-tween-px-r0-nested-z100).
• A move pressed right after Add keyframe: before the new tween reaches the preview, the move is routed as a plain-element move and saved as CSS translate, which the new keys then override (seqfastkeys-none-px-r0-root-z100, 40.8 px off).
• A drag pressed while a resize is still saving: the resize's teardown clears the drag's start attributes, so the drag's commit reads a missing base.

Also checks whether the Studio hold that pins a keyframed position tween's first key from t=0 changes authored tweens an edit did not touch.

Status

Red cases first; fixes follow on this branch.

[miguel-heygen]

Held until the PR it stacks on merges; one open PR per lane. Reopens then.