Document: Cmd+Alt+Shift+U switches to the next unit of measurement

The reference app switches the units of measurement with
Ctrl+Alt+Shift+U on Windows and Option+Shift+Command+U on macOS.
VectorCraft could set the document's units with document.setUnits or
a right-click on a ruler, but no command stepped through them and no
key did it.

document.switchUnits (Switch Units, default Cmd+Alt+Shift+U) sets the
document's units to the next one in the order a right-click on a
ruler lists them: Points, Picas, Inches, Millimeters, Centimeters,
Pixels, Feet & Inches, Meters, Yards, Feet, and from Feet back to
Points. Each press is one undo step, and the reply names the new
units. Keyboard Shortcuts lists it under Document.

The precise cursors shortcut-editor test used Cmd+Alt+Shift+U as a
free key; it now uses Cmd+Alt+Shift+Q.

Docs: docs/mcp.md, docs/ui-parity.md, docs/target-app-parity.md.

crates/engine/src/cmd/edit.rs

@@ -42,6 +42,15 @@ pub fn specs() -> Vec<CommandSpec> {
             has_doc,
             set_units
         ),
+        cmd!(
+            "document.switchUnits",
+            "Switch Units",
+            [],
+            Some("Cmd+Alt+Shift+U"),
+            "{} the document's units become the next of Points, Picas, Inches, Millimeters, Centimeters, Pixels, Feet & Inches, Meters, Yards and Feet (the order a right-click on a ruler lists them), Feet going back to Points; one undo step → {units}",
+            has_doc,
+            switch_units
+        ),
         cmd!(
             query "document.rulerOrigin",
             "Ruler Origin",
@@ -332,6 +341,14 @@ fn set_units(s: &mut Session, p: &Value) -> Result<Value> {
     ok()
 }
 
+/// Switch Units: the unit after the document's in [`Unit::ALL`], wrapping around.
+fn switch_units(s: &mut Session, _: &Value) -> Result<Value> {
+    let now = s.doc()?.doc.units;
+    let next = Unit::ALL.iter().position(|u| *u == now).and_then(|i| Unit::ALL.get((i + 1) % Unit::ALL.len())).copied().unwrap_or_default();
+    s.set_document_units(next)?;
+    Ok(json!({ "units": next.label() }))
+}
+
 /// The `artboard` param of the ruler origin commands: an artboard index, 0 when not given.
 fn artboard_param(s: &Session, p: &Value, c: &str) -> Result<usize> {
     let index = match p.get("artboard") {