Surface bodies: Patch, Stitch, Trim and Thicken

Kernel (surfaces):
- Bodies with an open shell are surface bodies (Body::is_surface).
- patch_region: a planar patch of a sketch region. patch_edges: a planar
  patch filling a closed loop of a body's edges.
- copy_faces: faces copied out as a surface.
- stitch: sew surfaces along coinciding edges, turning faces to agree; the
  result is a solid when it closes.
- trim: cut a surface by a plane or face and keep one side.
- thicken: offset planar and circular-cylindrical faces along their
  normals (one side or symmetric) into a solid. Plane copies slide;
  cylinder copies scale about the axis, exact for NURBS circles. Free
  edges get ruled walls. Faces meeting at a sharp edge are refused.

Doc features Patch, Stitch, Thicken (with operations) and SurfaceTrim;
engine commands FusionSurfacePatchCommand, FusionSurfaceStitchCommand,
FusionSurfaceThickenCommand and FusionSurfaceTrimCommand.

Tests (volumes):
- A 10x10 patch thickened by 2 gives 200, also symmetric.
- Half a cylinder side thickened outward gives half a ring; both halves
  inward give the full ring.
- Six faces of a box stitched back give 6000; an open box with its top
  patched from the edges, stitched, gives 6000.
- Trim keeps 30 or 70 of 100.
- Engine: patch, trim, thicken gives 140.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/doc/src/eval.rs

@@ -1815,6 +1815,8 @@ fn replay(doc: &Document, vals: &BTreeMap<String, Value>, f: &Feature, st: &mut
 
 #[path = "eval_more.rs"]
 mod more;
+#[path = "eval_surface.rs"]
+mod surface;
 
 /// 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.
@@ -1914,6 +1916,9 @@ fn eval_feature(doc: &Document, vals: &BTreeMap<String, Value>, f: &Feature, st:
         FeatureKind::Boss { .. } | FeatureKind::Lip { .. } | FeatureKind::SnapFit { .. } | FeatureKind::Rest { .. } => {
             more::plastic_eval(doc, vals, f, st)
         }
+        FeatureKind::Patch { .. } | FeatureKind::Stitch { .. } | FeatureKind::Thicken { .. } | FeatureKind::SurfaceTrim { .. } => {
+            surface::eval(doc, vals, f, st)
+        }
         FeatureKind::Emboss { .. }
         | FeatureKind::Rib { .. }
         | FeatureKind::ReplaceFace { .. }