Surface walls named after their sketch curves (#757) (#850)

Each wall of a surface built from sketch curves (Surface Extrude, Revolve, Sweep and Loft) is now named after the curve it comes from, exactly rather than from geometry, so references to those faces survive edits.

crates/kernel/src/build.rs

@@ -37,6 +37,12 @@ fn wire_sources(plane: &Plane, lp: &Loop2) -> Result<(mt::Wire, Vec<(mt::Edge, S
 /// An open wire in 3D through a chain of sketch segments, each starting where the one before
 /// ends (a closed chain makes a closed wire).
 pub(crate) fn chain_wire(plane: &Plane, segs: &[Seg2]) -> Result<mt::Wire> {
+    chain_wire_seeds(plane, segs).map(|(wire, _)| wire)
+}
+
+/// [`chain_wire`], with each edge's id and the position in the chain of the segment it was
+/// built from (a big arc makes two edges), in wire order.
+pub(crate) fn chain_wire_seeds(plane: &Plane, segs: &[Seg2]) -> Result<(mt::Wire, Vec<(mt::EdgeID, usize)>)> {
     let n = segs.len();
     if n == 0 || n > MAX_SEGS {
         return Err(KernelError::Invalid(format!("curve chain with {n} segments")));
@@ -50,7 +56,8 @@ pub(crate) fn chain_wire(plane: &Plane, segs: &[Seg2]) -> Result<mt::Wire> {
         }
     }
     let closed = segs.first().zip(segs.last()).is_some_and(|(f, l)| l.end().dist(f.start()) <= 1e-6);
-    chain_sources(plane, segs, closed).map(|(wire, _)| wire)
+    let (wire, sources) = chain_sources(plane, segs, closed)?;
+    Ok((wire, sources.into_iter().map(|(e, _, i)| (e.id(), i)).collect()))
 }
 
 /// The edges of a chain of segments (closed: the last one ends on the first's start vertex),