fix(doc): To Object takes the face at the pick, not the first body near it (#429)

object_height, curved_target and curved_target_any took the first solid body with any face
within its pick tolerance of the To Object point. That tolerance is 5 % of the body's size,
so a large body beside a small target took picks made on the target: a ridge extruded up
to a post 2 mm above a plate treated the plate as the target, nothing trimmed the tool, and
it ran past the post (to x 9.08 instead of -2.31).

They now share face_at: the nearest face over every solid body, still within that body's
tolerance. appearance gains nearest_face (the face and its distance) and pick_tolerance,
which face_index_at now uses as well.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/doc/src/appearance.rs

@@ -239,6 +239,12 @@ impl Document {
 
 /// The B-rep face of a body nearest a point (within 5% of the body's size).
 pub fn face_index_at(b: &crate::ModelBody, p: Vec3) -> Option<usize> {
+    let (d, f) = nearest_face(b, p)?;
+    (d <= pick_tolerance(b)).then_some(f)
+}
+
+/// The face of `b` nearest `p` and how far it is, however far.
+pub fn nearest_face(b: &crate::ModelBody, p: Vec3) -> Option<(f64, usize)> {
     let m = b.mesh();
     let mut best: Option<(f64, u32)> = None;
     for (t, f) in m.triangles.iter().zip(&m.tri_face) {
@@ -248,8 +254,12 @@ pub fn face_index_at(b: &crate::ModelBody, p: Vec3) -> Option<usize> {
             best = Some((d, *f));
         }
     }
-    let (d, f) = best?;
-    (d <= b.body.size() * 0.05 + 1e-6).then_some(f as usize)
+    best.map(|(d, f)| (d, f as usize))
+}
+
+/// How far from a face of `b` a picked point may lie and still pick it (`face_index_at`).
+pub fn pick_tolerance(b: &crate::ModelBody) -> f64 {
+    b.body.size() * 0.05 + 1e-6
 }
 
 /// The point of triangle abc nearest p.