M4.49: PNG-8 and GIF export with a palette quantizer and dithering

document.export {format: "png8" | "gif"} reduces the art to a palette of at most `colors`
(2–256) entries: median cut over a 15-bit histogram refined by k-means (perceptual,
selective or adaptive weighting), or the web-safe, black-and-white or grey palettes; art
with that many colours or fewer keeps them exactly. Dithering is Floyd–Steinberg diffusion,
an 8×8 ordered pattern or noise, with an amount. Pixels under half opacity become one
transparent entry; partly transparent edges blend over the matte. PNG-8 is an indexed PNG
(PLTE, tRNS, 1–8 bits per pixel, Adam7); GIF is a GIF89a (optionally interlaced). Both are in
document.formats, Export As (PNG-8 / GIF Options dialogs) and Export for Screens.
document.formats' writable list and the MCP export tool's format enum gain gif and png8
(their tests list them).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/fileio/export.rs

@@ -101,7 +101,7 @@ fn screen_format(row: &Value) -> Result<ScreenFormat> {
     }
     let format = writable(C, Some(str_param(row, "format").unwrap_or("png")), None)?;
     if matches!(format.id, "vectorcraft" | "template") {
-        return Err(bad(C, "Export for Screens writes png, jpg, webp, svg, svgz or pdf"));
+        return Err(bad(C, "Export for Screens writes png, jpg, webp, gif, png8, svg, svgz or pdf"));
     }
     // Vector formats have no pixel size: scale doesn't apply and adds no @Nx suffix (not even one
     // left over from a raster row switched to SVG or PDF).