Time > Pixel Motion Blur fetched the layer at its shutter times and averaged the decoded
frames as they were, so fast motion rendered as doubled copies and the accepted Vector
Detail parameter changed nothing. Build a TimewarpPlan instead: every shutter instant is
decoded between the two source frames it falls between along a measured motion field — the
same block-matching and in-between synthesis Time > Timewarp uses, with a search radius
wide enough for full-frame displacements and the vector grid tightened from Vector Detail
0 (one vector per 32 px) to 100 (one per 2 px) — and the instants average into the blur.
CPU and GPU share the plan (fx_time renders it with its Timewarp step loop, declining to
the CPU when there is no host), so the existing GPU parity cases for the effect compare
the new path. Falling back to the old average is kept for layers without a time host.
A render integration test moves a textured block 12 px between source frames, renders
the straddling shutter, and asserts the output is neither the still nor the plain frame
overlay, lays trail coverage along the block's path, and changes with Vector Detail;
without the synthesis it fails at the overlay assertion.
