feat(engine): sub-frame multi-sample motion blur for rotation, scale and opacity

Summary

The motion blur catalog component (#4008) integrates an element's translation over the shutter window: it reads the element's position at the start and end of the window and stacks N equal-weight copies along that path. That matches a 180 degree shutter reference within a pixel for elements that move across the frame.

It cannot blur change that is not a position change inside the same frame:

• rotation during the shutter window (a spinning logo has crisp edges),
• scale changes (a zoom pop has no radial smear),
• opacity ramps and colour changes,
• anything driven by a filter, clip-path, or child animation rather than a transform on the blurred element.

After Effects handles all of these with one mechanism: the layer is re-rendered at several sub-frame times inside the shutter window and the results are averaged.

Proposal

Add an engine-level accumulation mode to frame capture: for each output frame, seek the timeline to K sub-frame positions spread over the shutter window (default 180 degrees, phase -90 degrees, K configurable), capture each, and average the captures into the output frame.

• Opt-in per composition (root attribute, for example data-motion-blur-samples="8" and data-shutter-angle="180"), default off so render cost is unchanged for existing projects.
• Reuse the existing seek plumbing: the timeline is already sought per frame; this is K seeks per frame followed by a blend.
• Blend in linear light or at least document that the blend is in sRGB, so results match the component's behaviour.
• Preview/Studio can show the effect at a lower K or disable it, the way HDR preview already degrades.

Acceptance

• A spinning element renders with radial smear proportional to angular velocity; a scaling element renders with a radial zoom smear; an opacity ramp renders as the integrated opacity over the window.
• A translation-only composition rendered with the engine mode matches the catalog component output within the tolerance measured in #4008.
• Render time scales linearly with K and is unchanged when the mode is off.

Context

The catalog component stays as the cheap default for moving elements. The engine mode is what makes AE-style blur complete for rotation, scale, and fades, which are the cases people currently call "impossible on the web".