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 <[email protected]>

xtask/src/layers.rs

@@ -58,6 +58,7 @@ pub const TABLE: &[(&str, Class)] = &[
     // Crates added later (append-only).
     ("cad", Class::Layer(3)),
     ("eps", Class::Layer(3)),
+    ("metafile", Class::Layer(3)),
 ];
 
 /// Explicit orderings *within* a layer (earlier may be used by later).