face.delete heal: false leaves an open surface body (#861)

Delete Face takes heal (default true, as before). With heal: false the picked faces are removed and the gap stays open, leaving the remaining faces as a surface body (kernel remove_faces); deleting every face is refused. Saved only when false, noted under format 18.

crates/doc/src/document.rs

@@ -607,6 +607,10 @@ pub enum FeatureKind {
         faces: Vec<Vec3>,
         #[serde(default, skip_serializing_if = "Option::is_none")]
         body: Option<String>,
+        /// Close the gap by extending the neighbouring faces (the default); `false` leaves it
+        /// open, the faces left a surface body.
+        #[serde(default = "yes", skip_serializing_if = "is_yes")]
+        heal: bool,
     },
     /// A box around bodies, grown by a margin.
     BoundingSolid {
@@ -1745,6 +1749,14 @@ fn all_profiles() -> ProfileSel {
     ProfileSel::All
 }
 
+fn yes() -> bool {
+    true
+}
+
+fn is_yes(b: &bool) -> bool {
+    *b
+}
+
 fn zero_expr() -> String {
     "0".into()
 }