Split Body by a face of a body: FusionSplitBodyCommand tool {body, point}

The Split feature takes an optional face tool (FaceAt, the face through a
point on a named body) and splits with kernel split_body, the tool's
surface extended; otherwise the plane as before. Engine test: a box split
by a cylinder's side into the core and the rest.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/doc/src/document.rs

@@ -312,10 +312,13 @@ pub enum FeatureKind {
     ConstructionPlane {
         plane: PlaneRef,
     },
-    /// Split a body with a plane into two bodies.
+    /// Split a body into the parts on either side of a plane, or of a face of a body (the face
+    /// through `point`, its surface extended) when `tool` is set.
     Split {
         body: String,
         plane: PlaneRef,
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        tool: Option<FaceAt>,
     },
     /// Scale bodies about a point (uniform, or per axis).
     Scale {
@@ -630,6 +633,13 @@ pub enum FeatureKind {
     },
 }
 
+/// A face picked by a point on it, on a named body.
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct FaceAt {
+    pub body: String,
+    pub point: Vec3,
+}
+
 /// Triangles of one imported mesh body (millimetres).
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
 pub struct MeshData {