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 <[email protected]>

docs/face-attachments.md

@@ -14,6 +14,11 @@ Edited surfaces without retained construction provenance, ambiguous sources, and
 generators require an explicit fixed plane instead. A nearby or parallel face is never used
 to repair a new attachment automatically.
 
+The flat faces of a sheet metal body are the exception: they have no construction
+certificate, so a sketch on one attaches to its panel by name (`sheet:<body>:<panel>:<side>`)
+and finds it again after every rebuild, unfold and refold. If that panel no longer exists the
+sketch reports a lost attachment, as above. See the sheet metal guide.
+
 ## Create and repair
 
 Select a supported planar face and create a sketch. The desktop selection supplies the exact