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/engine/src/cmd/sketch_face.rs

@@ -79,6 +79,7 @@ fn capture_impl(s: &Session, value: &Value, cmd: &str, internal: bool) -> Result
         by_component.entry(doc.model_body_component(body)).or_default().push(body);
     }
     let mut chosen = None;
+    let mut sheet_face: Option<(String, Plane)> = None;
     let mut work = state.bodies.len().saturating_add(doc.occurrences.len());
     for (component, path, placement) in placements {
         let Some(bodies) = by_component.get(&component) else {
@@ -104,6 +105,22 @@ fn capture_impl(s: &Session, value: &Value, cmd: &str, internal: bool) -> Result
             let inverse = mat_inverse(&frame).ok_or_else(|| bad(cmd, "invalid selected face frame"))?;
             let local_point = apply_point(&inverse, point);
             let tolerance = (body.body.size() * 1e-4).max(1e-5);
+            // A flat face of a sheet metal body attaches to its panel: the exact plane comes from
+            // the panel's flat region and fold, and the sketch keeps the face's name to follow it.
+            if !fixed
+                && !internal
+                && let Some(sheet) = state.sheets.iter().find(|sheet| sheet.body == body.name)
+                && let Some((face, plane)) = sheet.panel_face_at(local_point, tolerance * 10.0)
+            {
+                if chosen.is_some() || sheet_face.is_some() {
+                    return Err(bad(cmd, "face point is ambiguous; pick an exact body and face index"));
+                }
+                let relative = mat_mul(&author_inverse, &frame);
+                let plane = Plane::new(apply_point(&relative, plane.origin), apply_vector(&relative, plane.x), apply_vector(&relative, plane.y))
+                    .ok_or_else(|| bad(cmd, "invalid sheet face plane"))?;
+                sheet_face = Some((solidcraft_doc::sheet::sheet_face_name(&body.name, &face), plane));
+                continue;
+            }
             let faces = body.body.faces(tolerance)?;
             let mesh = body.body.tessellate(tolerance)?;
             work = work.saturating_add(mesh.triangles.len().saturating_mul(if requested_index.is_some() { 1 } else { faces.len() }));
@@ -138,6 +155,12 @@ fn capture_impl(s: &Session, value: &Value, cmd: &str, internal: bool) -> Result
             }
         }
     }
+    if let Some((name, plane)) = sheet_face {
+        if chosen.is_some() {
+            return Err(bad(cmd, "face point is ambiguous; pick an exact body and face index"));
+        }
+        return Ok(PlaneRef::Face { plane, at: apply_point(&author_inverse, point), name: Some(name), anchor: None });
+    }
     let (body, index, path, plane) =
         chosen.ok_or_else(|| bad(cmd, "the selected planar face no longer exists; choose another face or an explicit fixed plane"))?;
     if fixed {