EPS: Use Artboards also writes a central file with every artboard

With Use Artboards and two or more artboards, EPS export now writes a
central file under the path's own name before the file of each artboard.
The central file contains the whole document, and its bounding box is
the visible art, as in an export without Use Artboards. An export of a
single chosen artboard still writes one file at the path's name, and
Export for Screens still writes one EPS per artboard. When no art shows,
only the artboard files are written.

Docs: the useArtboards params text, the MCP export tool's range text,
docs/mcp.md, docs/control-protocol.md and docs/target-app-parity.md.

docs/control-protocol.md

@@ -655,7 +655,9 @@ EPS Options: Export As… with the EPS format (`file.exportAs {format: "eps"}`,
 (`preserve`, `discard`), `flattenerPreset` (a built-in or saved preset's name), `includeLinkedFiles`, `thumbnails`,
 `cmykPostScript`, `compatibleGradients`, `level` (`2`, `3`), plus `path` and the artboard choice (`useArtboards`,
 `range`). `ui.dialog.confirm` checks them (a bad value or an unknown preset keeps the dialog open), remembers them for
-the next EPS Options and writes the file(s), asking for a path when there is none.
+the next EPS Options and writes the file(s), asking for a path when there is none. With Use Artboards and two or more
+artboards, it writes a central file of the whole document under the chosen name and one `{stem}_{artboard}.eps` per
+artboard.
 
 Data Recovery: at launch, copies left behind by a crash open the `recovery` dialog (never copies another running
 instance keeps, so instances started side by side for screenshots don't get it). Fields: `copies` (`[{file, title,