Guides: a ruler guide shows while dragged with Real-time Drawing off (#4646)

With Preferences > Performance > Real-time Drawing and Editing off, tool
previews are deferred to the release, so a guide dragged out of a ruler
(guide.add) or moved with a selection tool (guide.move) showed only its
position label until the button was released. Guide previews now always
apply as the guide is dragged: the guide is all such a drag shows.

#4646's units and guides lost on reopen don't reproduce for a native
file: a new millimetre document's ruler guides and units survive Save,
Close and Open in .vectorcraft, PDF, .ai and EPS (new regression test,
and live in the app with Background Save). An SVG saved without Preserve
Editing reopens in pixels with no guides, as the reporter saw.

crates/engine/src/tooling.rs

@@ -743,8 +743,9 @@ impl Session {
                 Action::Begin(label) => self.begin_interaction(&label)?,
                 Action::Preview(cmd, p) => {
                     // Performance › Real-time Drawing and Editing (#394): off, remember the latest
-                    // params and apply them once on Commit instead of every move.
-                    if !self.prefs.real_time_drawing {
+                    // params and apply them once on Commit instead of every move. A ruler guide
+                    // being dragged still shows: the guide is all there is to see (#4646).
+                    if !self.prefs.real_time_drawing && !matches!(cmd.as_str(), "guide.add" | "guide.move") {
                         self.clear_deferred_overlay_cache();
                         let st = self.doc_mut()?;
                         if let Some(it) = &mut st.interaction {