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>

ROADMAP.md

@@ -321,6 +321,8 @@ PCB / Package, Render, Animation, Simulation, Manufacture/CAM, Drawings.
   exact holes and retained islands, including the union of multiple selected profiles, and
   records complementary depth layers as pockets while retaining the stock gauge. Unsupported
   tapered or bend-crossing machining refuses atomically instead of restoring material later.
+  Sketches attach to a sheet's flat faces (base and flanges, either side) by panel, in the
+  flat pattern's coordinates, and follow it through rebuilds and unfold/refold.
 - Rolling-ball fillet accuracy: independent offset-cylinder tee references check surface
   distance, actual trimmed tessellation, supporting-cylinder contacts and tangency at two
   scales. The measured trimmed-part error is below 0.0001 mm; this is an analytic geometry