Snapshot the whole edit in the history, so a drawn mask can be taken back

The undo stack snapshotted a `Preset` — the parameter map — and a mask layer
is deliberately not a parameter. So drawing one changed nothing the history
could see: `record` returned `false`, no step opened, and the layer the
photographer had just painted had no way back. The interface went on calling
`record` in good faith, including from the mask controls, and nothing failed.
A film stock went missing the same way.

The snapshot is an `EditState` now, so the history is complete by
construction rather than by anyone keeping a list in their head.

`undo` and `redo` return a `Step` rather than a `bool`. Stepping is not the
only outcome a caller has to act on — a step across a change of film leaves
the graph without its tables, and only the caller can bake them — and a `bool`
would let that be dropped by writing nothing at all, which is the shape of
mistake this module had already made once. `DevelopSession` settles the debt
either way; a step that found nowhere to go is left alone, since clearing the
film because undo hit the floor would take the stock off the picture.

Five tests, all of which fail against the old snapshot: a drawn layer is
undoable and redoable, a layer's own settings are a step of their own, a
change of stock is a step and names what it needs baked back, clearing the
film is undoable, and an exposure move does not deep-copy the mask stack.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

core/dr-pipeline/tests/spots.rs

@@ -259,10 +259,10 @@ fn undo_takes_a_repair_back() {
     assert!(history.record(&graph, Edit::Discrete));
     assert_eq!(graph.spots().len(), 1);
 
-    assert!(history.undo(&mut graph));
+    assert!(history.undo(&mut graph).moved());
     assert_eq!(graph.spots().len(), 0, "the repair is still on the frame");
 
-    assert!(history.redo(&mut graph));
+    assert!(history.redo(&mut graph).moved());
     assert_eq!(graph.spots().len(), 1, "and redo could not put it back");
 }
 
@@ -287,7 +287,7 @@ fn undo_steps_back_through_a_moved_source() {
         .set_offset((0.2, 0.1));
     history.record(&graph, Edit::Discrete);
 
-    assert!(history.undo(&mut graph));
+    assert!(history.undo(&mut graph).moved());
     assert_eq!(
         graph.spots().get(&id).map(|s| s.offset),
         Some((0.08, 0.0)),