Dynamic comment field for value editing with validation feedback

- Comment shows "Enter=Save Esc=Cancel" when valid, "! value" when invalid
- Only updates on validation state change to avoid lag
- Uses deferred timer for Scintilla document sync
- Added tests for comment echo and validation error display

src/editor.h

@@ -88,6 +88,8 @@ private:
         int        linelenAfterReplace = 0;
         QString    original;
         NodeKind   editKind = NodeKind::Int32;
+        int        commentCol = -1;  // fixed comment column (stored at edit start)
+        bool       lastValidationOk = true;  // track state to avoid redundant updates
     };
     InlineEditState m_editState;