When render-wasm is active, shape exports (single file, ZIP, and Export
boards multi-page PDF) run on the shared worker WASM module so the UI
stays responsive. A progressive PDF session (begin/add-page/finish)
updates progress per page; cancel settles on the client. The server
exporter path remains for the SVG renderer.
Skia maps ligatures and other substituted glyphs to U+0000 in the PDF
ToUnicode maps. Rebuild those maps from the laid-out paragraphs and
append them as an incremental update.
Closes#12393
Headless raster/PDF used the board selrect so overflowing children stayed
cropped, but that also clipped the board's own drop shadow. Use
export_root_bounds: own strokes/shadows/blur without joining children.
Closes#12388
