Fix #569: Shape Builder merges leave no stray edges in their outlines

Merging regions united the faces' outlines with a boolean union. Each
face had been refit on its own, so two faces' shared edge could come out
a hair apart: the union then kept the edge between them, or a spur of
it, and a stroked merged shape (unfilled art) showed a stray line inside.
Subtracting the merged shape from the others left slivers along it too.

pathops::FaceMerger merges faces from the planar arrangement itself,
made once per merge: their contours share their edges exactly, so the
edges between merged faces vanish. The Shape Builder builds the merged
shape that way, and what is left of each shape from its faces not merged
(rather than subtracting the merged shape from it). Selections with open
paths keep the old union.

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

crates/engine/src/cmd/buildcmds.rs

@@ -250,7 +250,8 @@ fn sb_merge(s: &mut Session, p: &Value) -> Result<Value> {
         // Edges come back from the planar map within a few hundredths of the outlines they follow.
         let cut_tol = step * 0.2;
         let picked: Vec<&po::Region> = touched.regions.iter().filter_map(|&i| map.regions.get(i).map(|r| &r.0)).collect();
-        let union = if picked.is_empty() { PathData::default() } else { po::merge_regions(&picked) };
+        let faces = po::FaceMerger::new(&map.shapes);
+        let union = if picked.is_empty() { PathData::default() } else { faces.merge(&picked) };
         // The merged shape takes the look of the front-most object over the first region, or (for an
         // area only lines enclose) of the front-most object and the current fill.
         let covered = picked.iter().find_map(|r| r.top());
@@ -278,7 +279,17 @@ fn sb_merge(s: &mut Session, p: &Value) -> Result<Value> {
                 // its outline, shared edges from every shape along them).
                 let in_union = picked.iter().any(|r| r.sources.contains(&i));
                 let mut rest = if in_union {
-                    po::boolean(&sh.path, sh.rule, &union, FillRule::NonZero, po::BoolOp::Difference)
+                    // What is left of it: its faces not merged or deleted, united. They share their
+                    // edges with the merged faces exactly, where subtracting the merged shape left
+                    // slivers of the outline along them (#569).
+                    let kept: Vec<&po::Region> = map
+                        .regions
+                        .iter()
+                        .enumerate()
+                        .filter(|(k, (r, ..))| r.sources.contains(&i) && !touched.regions.contains(k))
+                        .map(|(_, (r, ..))| r)
+                        .collect();
+                    if kept.is_empty() { PathData::default() } else { faces.merge(&kept) }
                 } else {
                     b::node_outline(leaf).map_or_else(|| sh.path.clone(), |o| o.0)
                 };