Join walls swept along splines in 2D (#908)

Walls swept along a spline have flat B-spline tops and bottoms, so booleans between them went to the general intersection, which fails on coincident spline faces. Such faces are now put on true planes, and two prisms of the same height and direction are decided in 2D with their spline outlines fitted by arcs within a micron. The result is kept only when its volume is plausible; otherwise the general boolean takes over. The free-form flag is cleared by a drop guard so an unwinding boolean can't leave it set.

Fixes #844.

crates/kernel/src/prism.rs

@@ -485,7 +485,11 @@ impl Index {
         for w in f.boundaries() {
             let (mut segs, mut tags) = (Vec::new(), Vec::new());
             for e in w.edge_iter() {
-                let pieces = crate::prism_curve::segment(e, plane, tol, work)?;
+                let pieces = if FREE_FORM.with(|c| c.get()) {
+                    crate::prism_curve::segment_fitted(e, plane, tol, PLANAR_FIT_TOL, work)?
+                } else {
+                    crate::prism_curve::segment(e, plane, tol, work)?
+                };
                 *segments = segments.checked_sub(pieces.len())?;
                 let tag = self.tag(e, cap);
                 tags.extend((0..pieces.len()).map(|k| tag.piece(k, pieces.len())));
@@ -1919,6 +1923,28 @@ fn stack_indexed(a: &Body, b: &Body, op: BoolOp, [ia, ib]: [&LazyIndex; 2]) -> O
     sew_sections(&plane, d, &levels, &slabs, tol, coalesce, &history).map(|r| r.map(Some))
 }
 
+thread_local! {
+    /// Set while [`prism_boolean_free_form`] runs: spline outlines are read as arcs within
+    /// [`PLANAR_FIT_TOL`] instead of refusing the prism.
+    static FREE_FORM: std::cell::Cell<bool> = const { std::cell::Cell::new(false) };
+}
+
+/// [`prism_boolean`] for prisms whose walls are splines (swept along one): the outlines are
+/// fitted by arcs within [`PLANAR_FIT_TOL`], so the result is exact to that, and the walls it
+/// keeps are those arcs. Only for bodies the caller knows are such sweeps.
+pub(crate) fn prism_boolean_free_form(a: &Body, b: &Body, op: BoolOp) -> Option<Result<Option<Body>>> {
+    /// Clears the flag however the boolean ends, a caught panic included.
+    struct Reset;
+    impl Drop for Reset {
+        fn drop(&mut self) {
+            FREE_FORM.with(|c| c.set(false));
+        }
+    }
+    FREE_FORM.with(|c| c.set(true));
+    let _reset = Reset;
+    prism_boolean(a, b, op)
+}
+
 /// Boolean of prisms along a common direction, in 2D (see the module notes); `None` when it
 /// doesn't apply. The result carries its history over `[a, b]` (see [`lineage`]).
 pub(crate) fn prism_boolean(a: &Body, b: &Body, op: BoolOp) -> Option<Result<Option<Body>>> {