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/book/src/guide/sheet-metal.md

@@ -89,3 +89,15 @@ tool with varying depth is refused before changing sheet geometry. Pocket profil
 but a pocket crossing a bend zone cannot yet be refolded. Machining folded bend material,
 tapered sheet cuts and other unsupported 3D sheet Cut/Intersect operations report an error before changing
 the design; they cannot silently restore removed material during a later sheet rebuild.
+
+A sketch can be drawn on a flat face of the sheet: the base or a flange, on either side. Pick
+the face (or give `sketch.create` the point `plane: {face: [x, y, z]}`) and the sketch attaches
+to that panel instead of to a certified source face. It keeps the face's name,
+`sheet:<body>:<panel>:<top|bottom>` (the panel is `base` or the flange's feature tag), and finds
+the face again whenever the sheet is rebuilt: after a parameter change such as the thickness or
+a flange angle, through Unfold / Refold, after the file is reopened and when the body is
+renamed. Sketch coordinates are the flat pattern's, so a point drawn on a flange lies where the
+flat pattern and its DXF have it, and on the bottom face y runs the other way, as seen from
+below. When the face is gone (its flange is suppressed or deleted) the sketch reports a lost
+attachment instead of moving to another face; **Redefine Sketch Plane** attaches it to another
+face. `fixed: true` still freezes the picked plane in space.

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