feat: sketches on a sheet metal face attach to its panel (#474)

A sketch on a flat face of a sheet metal body was refused ("this face has no exact planar
source"), so the only way was a plane frozen in space. When the sheet changes (a flange's
angle, the thickness), the frozen sketch stays behind: a cut from it fails on the slanted
flange or leaves a blind pocket in the thicker sheet.

`sketch.create` with a face of a sheet metal body now attaches the sketch to the panel the
face belongs to. It keeps the face's name, `sheet:<body>:<panel>:<top|bottom>` (the panel is
the base or a flange's feature tag), and takes the panel's exact plane from its flat region
and fold each time the model is evaluated, so the sketch follows the sheet through parameter
changes, unfold and refold. Its coordinates are the flat pattern's (the bottom face's y
mirrored). A face that is gone is reported as a lost attachment, never replaced by the
nearest face. The attachment follows a renamed or copied body, shows as `sheet_face` in the
command result and as its own status in Redefine Sketch Plane. `fixed: true` still freezes.

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

crates/doc/src/eval.rs

@@ -3074,6 +3074,38 @@ fn smooth_chain(b: &ModelBody, picks: &[Vec3]) -> Vec<Vec3> {
     out
 }
 
+/// `plane` with the sheet face it names (see `sheet::plane_sheet_face`) replaced by that face's current
+/// plane in the frame of component `comp`, or an error when the face is gone: a lost
+/// attachment is reported, never moved to the nearest face.
+pub(crate) fn sheet_face_plane_in_frame(doc: &Document, st: &ModelState, comp: u64, plane: &PlaneRef) -> Result<PlaneRef> {
+    Ok(match plane {
+        PlaneRef::Face { name: Some(name), anchor: None, .. } if crate::sheet::plane_sheet_face(plane).is_some() => {
+            let lost = || DocError::Invalid(format!("lost face attachment: the sheet face `{name}` no longer exists; reattach the sketch to a face"));
+            let (body, face) = crate::sheet::sheet_face_plane(st, name).ok_or_else(lost)?;
+            if doc.occurrences.is_empty() && doc.body_offsets.is_empty() {
+                return Ok(PlaneRef::Custom { plane: face });
+            }
+            // The body's own coordinates into the component's frame, as `state_in_frame` places its bodies.
+            let source = st.body(&body).ok_or_else(lost)?;
+            let inverse = crate::mat_inverse(&doc.component_transform(comp)).ok_or_else(lost)?;
+            let m = crate::mat_mul(&inverse, &doc.component_transform(doc.model_body_component(source)));
+            let m = doc.model_body_offset(source).map(|offset| crate::mat_mul(&m, offset)).unwrap_or(m);
+            let placed = Plane::new(crate::apply_point(&m, face.origin), crate::apply_vector(&m, face.x), crate::apply_vector(&m, face.y));
+            PlaneRef::Custom { plane: placed.ok_or_else(lost)? }
+        }
+        PlaneRef::Offset { base, distance } => {
+            PlaneRef::Offset { base: Box::new(sheet_face_plane_in_frame(doc, st, comp, base)?), distance: distance.clone() }
+        }
+        PlaneRef::AtAngle { base, axis_origin, axis_dir, angle } => PlaneRef::AtAngle {
+            base: Box::new(sheet_face_plane_in_frame(doc, st, comp, base)?),
+            axis_origin: *axis_origin,
+            axis_dir: *axis_dir,
+            angle: angle.clone(),
+        },
+        other => other.clone(),
+    })
+}
+
 /// The plane of the body face a sketch sits on, found again: among planar faces with the
 /// picked plane's normal, the one nearest `at`. The picked frame moves along its normal onto it.
 fn face_plane(st: &ModelState, picked: &Plane, at: Vec3, name: Option<&str>) -> (Plane, Option<String>) {
@@ -3205,6 +3237,10 @@ fn eval_feature(doc: &Document, vals: &BTreeMap<String, Value>, f: &Feature, st:
             };
             let (plane, face_warning) = match (anchor, plane) {
                 (Some(_), plane) => (doc.resolve_attached_plane(vals, plane, f.component, st, &fst)?, None),
+                // A flat face of a sheet: the panel's exact plane, from its flat region and fold.
+                (None, plane) if crate::sheet::plane_sheet_face(plane).is_some() => {
+                    (doc.resolve_plane_in(vals, &sheet_face_plane_in_frame(doc, st, f.component, plane)?, 0, Some(&fst))?, None)
+                }
                 (None, PlaneRef::Face { plane: picked, at, name, anchor: None }) => face_plane(&fst, picked, *at, name.as_deref()),
                 (None, other) => (doc.resolve_plane_in(vals, other, 0, Some(&fst))?, None),
             };