kernel: an emboss's arcs run on their wall's own frame, and its seam is read from the circle's angles

Two readings of the SMLib record went wrong in emboss_cylinder:

- The footprint arcs were labelled on the sketch's radial frame. The assembly gives each new
  circle edge the NURBS form on its label's quarter grid, and the wall face keeps its own form on
  its frame's grid (a cylinder's seam lies at an odd angle, build::SEAM_ANGLE). The two grids
  differ by a non-quarter turn, so the arc's form is not a straight line in the face's (u, v)
  and no exact pcurve exists: "no exact pcurve of a NURBS on a spline surface". The arcs at the
  wall's radius are now labelled on the wall face's frame (form::on_frame), so their forms lie on
  the face's grid and the pcurve is exact.
- wall_angles took the edge's interval as the circle's angles. On an SMLib record the interval
  is on the edge's NURBS form (2πr long for a whole turn), so a wall spanned some 63 rad: a
  profile across the seam was accepted, and a wall of radius 0.1 was refused as off its seam.
  It now reads Edge::curve_interval.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/kernel/src/emboss.rs

@@ -9,7 +9,7 @@ use crate::numbered::NumberedRegion;
 use crate::offset::{Surf, face_edges, surf_of};
 use crate::{KernelError, Result, guard};
 use solidcraft_geom::brep::eval::CurveEval;
-use solidcraft_geom::brep::{Brep, Curve, Frame, Loop, Surface};
+use solidcraft_geom::brep::{Brep, Curve, Frame, Loop, Surface, form};
 use solidcraft_geom::{Loop2, Plane, Region2, Seg2, Vec2, Vec3};
 
 fn fail(s: &str) -> KernelError {
@@ -40,10 +40,11 @@ fn wall_angles(b: &Brep, face: usize, o: Vec3, axis: Vec3, radial: Vec3, tangent
         if z.dot(axis).abs() < 1.0 - 1e-9 {
             continue;
         }
-        // The circle's parameter is its angle: the wall spans the edge's interval about the
-        // angle of its middle (a whole turn, from the seam, for a wall in one piece).
-        let c = CurveEval::new(e.parameters()).ok()?;
-        let [t0, t1] = e.interval;
+        // The circle's parameter is its angle: the wall spans the edge's angles on the circle
+        // (`interval` is on its NURBS form where it has one) about the angle of its middle (a
+        // whole turn, from the seam, for a wall in one piece).
+        let c = CurveEval::new(&e.curve).ok()?;
+        let [t0, t1] = e.curve_interval()?;
         let am = angle(c.point(0.5 * (t0 + t1))?, o, axis, radial, tangent);
         let half = 0.5 * (t1 - t0);
         return Some((am - half, am + half));
@@ -327,9 +328,12 @@ fn emboss_impl(body: &Body, anchor: Option<Vec3>, plane: &Plane, regions: &[Regi
             let at = o + axis * p.y + (radial * (p.x / r).cos() + tangent * (p.x / r).sin()) * r;
             return Ok(EmbossOutcome::Anchor(fi, at));
         }
-        let wall = Wrap { o, axis, radial, tangent, r };
         let mut asm = Assembly::from_record(&rec)?;
         let selected = asm.faces.get_mut(fi).and_then(Option::take).ok_or_else(|| fail("face"))?;
+        // The footprint's arcs lie on the wall face and are labelled on its frame, so their NURBS
+        // forms run on the face's own quarter grid and their pcurves there are exact.
+        let label = form::frame_of(&selected.surface);
+        let wall = Wrap { o, axis, radial, tangent, r, label, tol };
         // The wall's pieces left around the footprints: the face itself, then one per profile
         // hole (the original radius inside it).
         let mut base_patches: Vec<Vec<Vec<AUse>>> = vec![selected.loops.clone()];
@@ -416,6 +420,9 @@ struct Wrap {
     radial: Vec3,
     tangent: Vec3,
     r: f64,
+    /// The wall face's own frame, which the arcs at its radius are labelled on.
+    label: Option<Frame>,
+    tol: f64,
 }
 
 impl Wrap {
@@ -441,7 +448,12 @@ impl Wrap {
         let (ta, tb) = (p.x / self.r, q.x / self.r);
         let forward = ta < tb;
         let (interval, ends) = if forward { ([ta, tb], (a, b)) } else { ([tb, ta], (b, a)) };
-        Ok(AUse { edge: asm.edge(Curve::Circle { frame, radius }, interval, ends.0, ends.1)?, forward, given: None })
+        let circle = Curve::Circle { frame, radius };
+        let (circle, interval) = match &self.label {
+            Some(label) if (radius - self.r).abs() <= self.tol => form::on_frame(&circle, interval, label, self.tol).unwrap_or((circle, interval)),
+            _ => (circle, interval),
+        };
+        Ok(AUse { edge: asm.edge(circle, interval, ends.0, ends.1)?, forward, given: None })
     }
 
     /// A profile loop wrapped onto the wall: its footprint at the wall's radius and its cap's