M4.77: DXF export (R12 to 2018) with DXF Options; DWG and PICT say what to use instead

New crates/cad (L3): a hand-written ASCII DXF writer. Layers become DXF
layers (hidden: off, locked, non-printing: not plotted), straight paths
LWPOLYLINE (POLYLINE before R14), curves cubic SPLINEs through every
anchor, fills solid HATCHes (outlines in R12), strokes lines with a
lineweight and a linetype per dash pattern, type TEXT or glyph outlines,
placed images IMAGE entities linked to PNG or JPEG files. Units and
scale ($INSUNITS), scaled lineweights, 8/16/256 indexed or true colour
(420), transparency (440), Preserve Appearance / Maximize Editability,
Alter Paths for Appearance, Outline Text.

document.export {format: dxf} and document.exportDxf; useArtboards
writes one drawing per artboard; selectedOnly (any export) writes the
selection in its layers. Export As lists DXF (then DXF Options,
ui.dxfOptionsDialog for agents, remembered between exports) and DWG
greyed out; document.formats lists DWG and PICT as unsupported with a
hint, and exporting or opening them answers with it.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

xtask/src/layers.rs

@@ -55,6 +55,8 @@ pub const TABLE: &[(&str, Class)] = &[
     ("cli", Class::Exempt),
     ("web", Class::Exempt),
     ("xtask", Class::Exempt),
+    // Crates added later (append-only).
+    ("cad", Class::Layer(3)),
 ];
 
 /// Explicit orderings *within* a layer (earlier may be used by later).