feat(kernel): an edge's meeting sides are sets of origins (merged faces)

EdgeOrigin::Meet now names each side as a Side: the sorted, de-duplicated
origins of the face there. substitute maps a side to the union of the inner
faces' origins, so a meeting with a face an inner operation merged keeps its
lineage; meeting() derives sides from whole faces. Fillets after box cuts
that leave coplanar pieces now report exact history. Prism lineage meets a
cap of several origins and resolves wall junctions away from shared
vertices by the operand's one edge between the walls. bore_union's recut
reports history only for a one-face bore (pieces of a two-arc bore's recut
wall were given both arcs).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J53sR6gEaVt56soFDpTck8

crates/kernel/src/polybool.rs

@@ -587,7 +587,7 @@ impl Sources {
     fn edge(&self, p: FaceOrigin, q: FaceOrigin, ends: (Vec3, Vec3), tol: f64) -> Option<Vec<EdgeOrigin>> {
         let (FaceOrigin::Face { operand: x, face: f }, FaceOrigin::Face { operand: y, face: g }) = (p, q) else { return None };
         if x != y {
-            return Some(vec![EdgeOrigin::Meet { a: p.min(q), b: p.max(q) }]);
+            return Some(vec![EdgeOrigin::meet(p, q)]);
         }
         let Some(edges) = self.edges.get(x)?.as_ref() else { return Some(Vec::new()) };
         if f == g {