kernel: a cut's tool face blocks a planar role only where it enters the result

map_planar_sources took every plane of a cut's tool as competing with an input face on it,
facing either way, and dropped that face's exact role. A text pocketed 1 mm down from a block's
top has its tool's top on the top's plane facing up, as the top does; the top then lost its
CertifiedExtrudeCap, and the sketch on it reported "the top cap of Extrude1 no longer exists"
(a_face_sketch_saved_with_fitted_letters_reopens_and_builds_the_same). A cut's tool faces enter
the result turned round, as the walls of the hollow, so a tool face can only take the place of a
face on its plane facing against it (a pocket's floor, a face the tool sat on from outside). Only
that orientation competes now; the existing blocking cases (a tool's bottom on a pocket floor, a
tool face against a wall) are unchanged.

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

crates/kernel/src/planar_sources.rs

@@ -225,9 +225,13 @@ impl Body {
         let mapped = guard("exact planar support lineage", || {
             let mut competing = Vec::new();
             for plane in blocker.map(actual_planes).transpose()?.into_iter().flatten().flatten() {
-                // A tool face blocks a face on its plane facing either way.
+                // A cut's tool face enters the result turned round (the wall of the hollow it
+                // leaves), so it competes only with a face on its plane facing against it: a
+                // pocket's floor, or a target face the tool sat on from outside. A tool face that
+                // faces the same way as the target's face on its plane (a pocket cut down from a
+                // cap, its top on the cap) is the tool's own outside and leaves the cap the cap.
                 let opposing = Plane::new(plane.origin, plane.x, -plane.y).ok_or_else(|| KernelError::Invalid("invalid opposing plane".into()))?;
-                competing.extend([plane, opposing]);
+                competing.push(opposing);
             }
             competing.sort_by(|a, b| a.normal().x.total_cmp(&b.normal().x));
             let mut candidates = Vec::new();