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/engine/src/cmd/sketch_face_tests.rs

@@ -76,7 +76,8 @@ fn explicit_rebind_preserves_geometry_dimensions_context_and_one_undo() {
 fn unsupported_face_has_explicit_fixed_alternative_and_no_live_projection() {
     let mut s = Session::default();
     run(&mut s, "solid.box", json!({"length":10,"width":10,"height":10,"body_name":"Base"}));
-    run(&mut s, "solid.box", json!({"length":2,"width":2,"height":2,"corner":[4,4,10],"operation":"join"}));
+    // The tops of two joined boxes merge into one face that no single construction owns.
+    run(&mut s, "solid.box", json!({"length":10,"width":10,"height":10,"corner":[10,0,0],"operation":"join"}));
     let mut pick = top_pick(&s, "Base");
     let before = s.doc.to_json();
     let error = s.execute("sketch.create", &json!({"plane":pick})).unwrap_err();