Video-only (no-audio) H.264 export now uses the same background-thread zero-copy
VAAPI path as video+audio, instead of the software encoder. It writes the output
.mp4 directly (no mux step) and reports through the single-export progress
channel; completion clears that channel so the dialog closes cleanly.
Factored the encoder/renderer/resources setup shared by both entry points into
ExportOrchestrator::try_build_zero_copy. start_video_export gains the document/
video_manager/raster_store/container_path inputs to seed the off-thread render
from a Document snapshot. Non-H264 / non-VAAPI still falls back to software.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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