Join keeps exact face attachments on faces it leaves unchanged (#420) (#453)

A Union dropped every construction certificate, so after any join no face
could take a durable sketch: sketch.create failed with "this face has no
exact planar source" even on a face the join never touched (#420, and the
"CREATE SKETCH: operation failed" reports #344, #195).

- kernel: map planar sources through a Union from both operands, with the
  same exact-support rule Cut uses. Coplanar supports of the two operands
  carry distinct identities, so a face merged from both stays without one.
- doc: Join extrude and Join box tools are certified like new bodies; a
  cap moved off its designed height (into the target, or past a curved To
  face) is not certified. Join producers are accepted when certifying.

Tests: both operands' faces attach and follow an upstream edit and a
reopen; a merged coplanar face is refused and fixed:true works; a face a
join covers whole is lost without moving to the new roof. Tests that used
a joined face as their "uncertified" example now use a merged face; the
trimmed-face case in a_sketch_on_a_face_stays_on_that_face now stays
attached to the surviving piece.


Claude-Session: https://claude.ai/code/session_01YDDPmNdRoBgkHcLeUDKVfb

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

crates/doc/src/config_shell_tests.rs

@@ -529,7 +529,15 @@ fn saved_stepped_tab_cut_piece_projection_is_lost_without_expanding_material() {
     let stem = crate::naming::strip_piece(&former);
     let current_names = crate::naming::face_names(current);
     let merged = current_names.iter().position(|name| name == stem).unwrap();
-    assert!(current.body.face_was_coalesced(merged), "new exact generated-wall construction supplies the loss-only witness");
+    // The Join leaves Case's -x wall on its own support, so the wall keeps its exact box
+    // identity through the Join and the Cut: one face again, no longer the saved piece.
+    assert!(
+        matches!(
+            current.body.face_sources().and_then(|sources| sources.get(merged)).cloned().flatten(),
+            Some(solidcraft_kernel::FaceSource::Box { axis: 0, high: false, .. })
+        ),
+        "the merged wall carries its exact construction identity"
+    );
     assert!(!current_names.contains(&former));
     let mut sketch = Sketch::new();
     sketch.add_link(LinkKind::Project, LinkSource::Face { body: "Case".into(), at }, &resolved.geom).unwrap();