.ai save: exports to a path and timestamped copies name the active artboard

A .ai file that File > Export As, its dialog or a control request writes
to a path now names the view's active artboard, as document.export and
document.serialize do. The UI export records the view's artboard and
takes its document from fileio::export_doc, which applies
DocState::active_artboard to the export's copy; document.export and
document.serialize use the same function.

The app now records the view's active artboard before every command,
including the commands the UI handles itself, so a .ai copy from Save a
Timestamped Copy or Export All Open Documents names it too. Before,
these files named the artboard the document opened on, or the first.
The open document is unchanged.

crates/engine/src/cmd/fileio.rs

@@ -57,9 +57,9 @@ use serde_json::{Value, json};
 pub use dxf::{UNSUPPORTED, Unsupported, unsupported};
 pub use encode::{ARTBOARD_PARAMS, ArtboardPick, Encoded, encode, encode_all, encode_with_warnings};
 pub(crate) use encode::{anti_alias, background, with_single_artboard};
-pub use export::export_source;
 pub(crate) use export::with_artboard_fills;
 pub(crate) use export::{export_crop, isolated};
+pub use export::{export_doc, export_source};
 pub(crate) use load::check_not_lossy_overwrite;
 use load::err;
 pub(crate) use load::import_svg;