History: the panel menu clears the history

The History panel menu had no Clear History, and VectorCraft had no
command that removes the undo and redo steps. In the reference app,
Clear History in the History panel menu removes the states after the
selected state, and it removes the states before it while their
changes remain in the document.

edit.clearHistory (Clear History) removes every step to undo and to
redo in the active document and returns how many it removed. The
document and the selection stay as they are, and clearing is not an
undo step. Typing or a drag still in progress becomes a step first, as
Undo makes it one, so its change stays in the document. The command is
disabled when there is no step to clear, and it waits while an input
method composes, as Undo does.

The History panel menu has Clear History after Step Forward. It runs
the command and is disabled when there is no step to clear.

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

crates/engine/src/cmd/edit.rs

@@ -75,6 +75,15 @@ pub fn specs() -> Vec<CommandSpec> {
         ),
         cmd!("edit.undo", "Undo", ["Edit"], Some("Cmd+Z"), "{}", can_undo, undo),
         cmd!("edit.redo", "Redo", ["Edit"], Some("Cmd+Shift+Z"), "{}", can_redo, redo),
+        cmd!(
+            "edit.clearHistory",
+            "Clear History",
+            [],
+            None,
+            "{} remove every undo and redo step of the active document (the History panel menu's Clear History): the document stays as it is, and clearing is not an undo step → {cleared: the number of steps removed}",
+            can_clear_history,
+            clear_history
+        ),
         cmd!(
             "edit.undoGroup.begin",
             "Begin Undo Group",
@@ -418,6 +427,19 @@ fn redo(s: &mut Session, _: &Value) -> Result<Value> {
     Ok(json!({ "redone": label }))
 }
 
+/// History panel › Clear History: removes the steps after the current state and the steps before
+/// it, and leaves the document as it is. Typing or a drag still in progress is first kept as a
+/// step, as Undo keeps it, so it is removed with the others and its change stays in the document.
+fn clear_history(s: &mut Session, _: &Value) -> Result<Value> {
+    settle_interaction(s)?;
+    let st = s.doc_mut()?;
+    let cleared = st.history.undo.len() + st.history.redo.len();
+    st.history.undo.clear();
+    st.history.redo.clear();
+    st.revision += 1;
+    Ok(json!({ "cleared": cleared }))
+}
+
 /// `selection` for an undo (redo) step, sharing that of `like`, the step beside it, when the same.
 fn shared_selection(selection: vectorcraft_doc::Selection, like: Option<&crate::HistoryEntry>) -> Arc<vectorcraft_doc::Selection> {
     match like {

crates/engine/src/cmd/mod.rs

@@ -181,6 +181,10 @@ pub fn can_undo(s: &Session) -> std::result::Result<(), String> {
 pub fn can_redo(s: &Session) -> std::result::Result<(), String> {
     s.active().filter(|d| !d.history.redo.is_empty()).map(|_| ()).ok_or_else(|| "nothing to redo".into())
 }
+/// The active document has a step to undo or to redo (Clear History).
+pub fn can_clear_history(s: &Session) -> std::result::Result<(), String> {
+    s.active().filter(|d| !d.history.undo.is_empty() || !d.history.redo.is_empty()).map(|_| ()).ok_or_else(|| "no history to clear".into())
+}
 pub fn has_clipboard(s: &Session) -> std::result::Result<(), String> {
     has_doc(s)?;
     if s.clipboard.is_empty() { Err("clipboard is empty".into()) } else { Ok(()) }