Variables: Update Data Set writes only the variables the active data set holds

A data set can hold values for only some variables: `dataset.new` and
`dataset.set` store only the values they are given, and selecting the
data set leaves the other variables alone. Update Data Set replaced the
active data set's values with the values the art reports for every
variable. After one Update, a data set that set only a visibility
variable also held the current text of the bound type, and selecting it
again wrote that text back. Update also dropped a variable's value when
its objects were locked or inside a hidden or locked layer or group,
because such objects report no value.

`dataset.update` now writes the art's current value for each variable
the active data set holds, and keeps the data set's value when the art
reports none. A variable that the data set does not hold stays out of
it, including one defined after the data set was captured;
`dataset.update {all: true}` also adds the other variables that the art
reports a value for. The reply adds `left`, the variables it did not
write. The Variables panel's Update Data Set button and menu item run
it without `all`.

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

crates/engine/src/cmd/variablecmds.rs

@@ -5,7 +5,7 @@
 //! dataset, and `dataset.select` swaps the whole document to that row in one undo
 //! step. Agents reach every command through `run_command`, like all engine commands.
 
-use std::collections::BTreeMap;
+use std::collections::{BTreeMap, BTreeSet};
 
 use serde_json::{Value, json};
 use vectorcraft_doc::{DataSet, DataValue, NodeId, NodeKind, Variable, VariableKind, Variables};
@@ -327,10 +327,14 @@ fn dataset_capture(s: &mut Session, p: &Value) -> Result<Value> {
     Ok(json!({"name": name, "values": values.len()}))
 }
 
-/// Update Data Set: write what the bound objects hold now into the row that is active. Without
-/// an active row there is nothing to update, so say so rather than picking one.
-fn dataset_update(s: &mut Session, _p: &Value) -> Result<Value> {
+/// Update Data Set: write what the bound objects hold now into the row that is active, for each
+/// variable the row holds. A variable whose objects report no value (locked, inside a hidden or
+/// locked layer or group, or gone) keeps the row's value. With `all`, every other variable the
+/// objects report a value for is added to the row. Without an active row there is nothing to
+/// update, so say so rather than picking one.
+fn dataset_update(s: &mut Session, p: &Value) -> Result<Value> {
     const C: &str = "dataset.update";
+    let all = bool_or(p, "all", false);
     let values = s.doc()?.doc.captured_values();
     let Some(name) = s.doc()?.doc.variables.active_dataset.clone() else {
         return Err(bad(C, "no active data set"));
@@ -338,12 +342,19 @@ fn dataset_update(s: &mut Session, _p: &Value) -> Result<Value> {
     if values.is_empty() {
         return Err(bad(C, "bind something to a variable first"));
     }
-    s.edit("Update Data Set", |d, _| {
+    let (written, left) = s.edit("Update Data Set", |d, _| {
         let ds = d.variables.datasets.iter_mut().find(|d| d.name == name).ok_or_else(|| bad(C, format!("no dataset named `{name}`")))?;
-        ds.values = values.clone();
-        Ok(())
+        let mut written = BTreeSet::new();
+        for (var, value) in &values {
+            if all || ds.values.contains_key(var) {
+                ds.values.insert(var.clone(), value.clone());
+                written.insert(var.as_str());
+            }
+        }
+        let left: Vec<String> = d.variables.variables.iter().filter(|v| !written.contains(v.name.as_str())).map(|v| v.name.clone()).collect();
+        Ok((written.len(), left))
     })?;
-    Ok(json!({"name": name, "values": values.len()}))
+    Ok(json!({"name": name, "values": written, "left": left}))
 }
 
 fn dataset_delete(s: &mut Session, p: &Value) -> Result<Value> {
@@ -544,7 +555,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Update Data Set",
             ["Window", "Variables"],
             None,
-            "{} write what the bound objects hold now into the active dataset",
+            "{all?: bool} write what the bound objects hold now into the active dataset, for each variable the dataset holds; a variable whose objects report no value (locked, inside a hidden or locked layer or group, or gone) keeps the dataset's value. `all: true` also adds every other variable the objects report a value for; `left` names the variables it didn't write → {values, left}",
             has_doc,
             dataset_update
         ),