feat(editor): capture_pie_movement_clip include_ui — composited game+UI frames

[whalemenace]

capture_pie_movement_clip frames are scene-only — the viewport ReadPixels path never sees UMG/Slate. If you're iterating on UI (which is half of what agents point cameras at), the clip shows an empty scene while the thing you care about is invisible. Our workaround was a hand-rolled python slate-tick ticker spamming shot showui and cropping the result, which nobody should have to write twice.

Changes:

• include_ui param (default false, no behavior change): frames are captured through the engine screenshot path (FScreenshotRequest with show-UI — the shot showui mechanism), so Slate composites the full backbuffer including UMG before the file is written.
• The write is end-of-frame async, so the per-frame uniformity validation doesn't apply to these frames; they count as requested and the clip directory holds the truth. Documented in the schema.
• Caveat, also documented: with PIE docked in the editor the composited backbuffer is the whole editor window; new-window PIE gives a clean game+UI clip.

Verified on 0.22.0: a 4s include_ui clip of a UMG-heavy map produced 7 frames with the full widget stack visible mid-animation — the exact frames the scene-only path renders empty.