M4.94: EMF and WMF export from Export As and document.export

New crates/metafile (L3, pure Rust): hand-written EMF and WMF writers over
one shared document walk. EMF keeps Bezier paths, solid brushes, geometric
pens (width, caps, joins, miter limit, user-style dashes), clipping masks as
path clips, images (StretchDIBits, or AlphaBlend with transparency) placed by
a world transform, type as outlines; gradients become images clipped to their
shape and pattern fills their tiles clipped to it. The header's frame is the
artboard in 0.01 mm on a 100 px/mm reference device. WMF flattens curves into
polygons in 16-bit units behind a placeable header (with its checksum),
numbers objects as players do, and reports what it leaves out (clipping,
transparency, gradients, rotated images).

document.export {format: emf|wmf} (and serialize, export) writes one artboard,
or one file per artboard with useArtboards (false: the art's bounds); raster
effects are rendered to images first, as for PDF. Export As lists EMF and WMF.

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

docs/control-protocol.md

@@ -361,3 +361,6 @@ Native save options: File → Save As to a native or `.ai` file opens the `saveO
 artboard; false: `range`, such as "1-3, 5"), `includeLinked`, `embedProfiles`, `pdfCompatible`, `compress`, and for
 native files `version` and `preview`. A bad range keeps the dialog open; `ui.dialog.confirm` writes the file and the
 artboards' files.
+
+EMF and WMF: Export As… lists both formats (`file.exportAs {format: "emf"}` or `"wmf"`); they have no options
+dialog, so `ui.dialog.confirm` asks for the path and writes the file (one per artboard with Use Artboards).