View state: hide, show and isolate viewport items, saved with the model

view.hide, view.show, view.isolate and view.show_all change which
bodies, surfaces, datums, boundary conditions, loads, reference points
and couplings the viewport draws. The hidden items are view state: the
manifest's `view` keeps them, a change marks the model unsaved, but it
is neither journalled nor undone and leaves results current. Items that
go away are forgotten. Headless pictures leave hidden items out; the
glyph builders take a filter. Tools > View > Show All claims the
commands.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/cmd/mod.rs

@@ -24,6 +24,7 @@ pub(crate) mod seeds;
 pub(crate) mod solve;
 mod symmetry;
 mod tags;
+pub(crate) mod view;
 
 use crate::schema::ProblemKind;
 use fieldcraft_model::MAX_LABEL;
@@ -47,6 +48,10 @@ pub static ALL: &[CommandSpec] = &[
     datum::DATUM_AXIS_EDIT,
     datum::DATUM_POINT_EDIT,
     datum::DATUM_DELETE,
+    view::HIDE,
+    view::SHOW,
+    view::ISOLATE,
+    view::SHOW_ALL,
     datum::PARTITION,
     datum::PARTITION_DELETE,
     mesh::NODE_ADD,