M4.57: PNG, PDF and text clipboard flavours in and out

Copy and Cut also offer other apps a PNG of the copied objects, a PDF
with On Copy: PDF, and plain text when every copied object is type
(clipboard.flavours; clipboard.exportPng {scale?}, exportPdf,
exportText). Paste outside text editing takes what another app copied:
SVG, then PDF (clipboard.importPdf), then plain text as point text
(clipboard.importText), then a bitmap embedded at the view centre
(clipboard.importImage {dataBase64, mime?, center?}); pasting our own
copy keeps the lossless internal clipboard, and Cmd+V pastes a bitmap
that comes with no text. Services gains system_clipboard
(SystemClipboard: write, holds_ours, read, has): on Windows every
format at once, on macOS and Linux one (the text, else the PNG); the
web keeps SVG text only. With When pasting text: Keep Plain Text, text
the Type tool copied pastes without its formatting.

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

crates/engine/src/cmd/fileio/encode.rs

@@ -181,7 +181,11 @@ fn use_artboards(p: &Value) -> Result<Option<bool>> {
 
 /// A copy of `doc` with one artboard, `rect` (exports of the art's or the selection's bounds).
 pub(super) fn single_artboard(doc: &Document, rect: Rect, name: &str) -> Document {
-    let mut d = doc.clone();
+    with_single_artboard(doc.clone(), rect, name)
+}
+
+/// `d` with one artboard, `rect`.
+pub(crate) fn with_single_artboard(mut d: Document, rect: Rect, name: &str) -> Document {
     d.artboards = vec![Artboard { id: 1, name: name.into(), rect, show_center_mark: false, show_cross_hairs: false }];
     d
 }