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/place/mod.rs

@@ -110,7 +110,7 @@ struct Loaded {
 }
 
 /// Points per pixel of a raster image at the resolution it declares (72 ppi when none).
-fn pt_per_px(ppi: Option<(f64, f64)>) -> (f64, f64) {
+pub(crate) fn pt_per_px(ppi: Option<(f64, f64)>) -> (f64, f64) {
     let (x, y) = ppi.unwrap_or((72.0, 72.0));
     (72.0 / x, 72.0 / y)
 }