fix(ui): the Combine dialog previews without an error, so OK is enabled (#481)

The live preview also evaluates each command as a new body, to show the tool of a join or cut
(as_new_body sets every `operation` to "new"). A combine has no new-body form: its operation is
between bodies that already exist, and solid.combine rejects "new" ("operation must be join, cut
or intersect"). That error became the preview's error even when the combine itself worked, and
since OK is greyed while the preview has an error, a combine could not be applied from its
dialog at all. as_new_body now leaves solid.combine alone, so its preview shows the result.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/src/preview.rs

@@ -276,7 +276,13 @@ fn build_tool(before: &ModelState, after: &ModelState, c: Colors, op: &str) -> B
 fn as_new_body(cmds: &[(String, Value)]) -> Option<(Vec<(String, Value)>, String)> {
     let mut out = cmds.to_vec();
     let mut changed: Option<String> = None;
-    for (_, p) in &mut out {
+    for (command, p) in &mut out {
+        let inner = p.get("command").and_then(Value::as_str).map(str::to_string);
+        // A combine's operation is between bodies that already exist: it has no new-body form
+        // (and no tool body of its own to show).
+        if inner.as_deref().unwrap_or(command.as_str()) == "solid.combine" {
+            continue;
+        }
         let p = if p.get("command").is_some() { p.get_mut("params")? } else { p };
         if let Some(op) = p.get_mut("operation")
             && let Some(o) = op.as_str().filter(|o| *o != "new").map(str::to_string)
@@ -455,6 +461,22 @@ mod tests {
         assert_eq!(pv.error.as_deref(), Some("current invalid expression"), "current errors still prevent invalid Apply");
     }
 
+    /// Combine previews its result (cut, join or intersect between existing bodies) without an
+    /// error, so its dialog's OK is enabled. Before, the preview also tried the command as a new
+    /// body, which a combine has no form of, and always reported "operation must be join, cut or
+    /// intersect".
+    #[test]
+    fn combine_previews_its_result_without_an_error() {
+        let mut s = solidcraft_engine::Session::default();
+        s.execute("solid.box", &json!({"length": 20, "width": 20, "height": 20, "body_name": "Cube"})).unwrap();
+        s.execute("solid.cylinder", &json!({"base": [10, 10, -5], "diameter": 6, "height": 30, "body_name": "Pin"})).unwrap();
+        for op in ["cut", "join", "intersect"] {
+            let cmd = ("solid.combine".to_string(), json!({"target": "Cube", "tools": ["Pin"], "operation": op, "keep_tools": false}));
+            let built = compute(&s, &[cmd], colors()).unwrap_or_else(|e| panic!("{op}: {e}"));
+            assert!(built.error.is_none(), "{op}: {:?}", built.error);
+        }
+    }
+
     fn count(bytes: &[u8], stride: usize, col: [u8; 4]) -> usize {
         bytes.chunks(stride).filter(|v| v.get(24..28) == Some(&col[..])).count()
     }