Let a part be left out of a mask without being taken out of it

A layer built from parts was missing the one control a correction most
often wants: seeing what it did. The question a subtracted gradient
raises is whether it took only the sky, and the question a stroke raises
is whether it filled the shoulder — and the only way to ask either was
to remove the part and look, which answered the question and lost the
part. The layer's own ring answers a different question, about the
adjustment, and hiding eight layers to check one correction is not an
A/B anybody performs.

So a part carries `hidden`. It is an edit and a history step, as the
layer's switch is, and it is folded into the render fingerprint because
hiding a part changes the mask as surely as removing it does. Where the
mask is built the shown parts are walked rather than the parts, which
is what makes a hidden base hand the fold to the first part that is
shown — and a revealed layer whose every part is hidden clears its slice
rather than leaving whatever the last rasterisation put there to be
read back. `covers` asks the same shown parts, so a layer whose only
adding part is hidden costs no slice at all.

In the sidecar the key is `hidden`, in the part's block or, for the
base, in the mask block — under a word that cannot be confused with the
layer's `enabled`, which has always meant the layer. Absent means shown,
so no file written before the switch existed reads any differently.

The row wears the same ring the layer does, one row down, because it is
the same question about a smaller thing.

core/dr-pipeline/tests/mask_sidecar.rs

@@ -1344,6 +1344,48 @@ fn a_correction_painted_onto_a_subject_survives_a_round_trip() {
     );
 }
 
+/// TRACES: FR-DEV-19a
+/// A part left out of the build comes back left out, and the file says so
+/// under a word that cannot be confused with the layer's own switch.
+#[test]
+fn a_hidden_part_stays_hidden_across_a_round_trip() {
+    let mut graph = EditGraph::default_chain();
+    let mut layer = corrected("m1", Join::Subtract);
+    layer.part_mut(1).expect("the correction").hidden = true;
+    graph.masks_mut().push(layer);
+
+    let mut sidecar = Sidecar::new();
+    sidecar.put(Version::from_graph("default", "Default", &graph));
+    let text = sidecar.to_text();
+    assert!(text.contains("hidden = 1"), "{text}");
+    assert!(
+        !text.contains("enabled = 0"),
+        "a hidden part must not read as a disabled layer:\n{text}"
+    );
+
+    let restored = round_trip(&graph);
+    let layer = &restored.masks().layers()[0];
+    assert!(layer.parts()[1].hidden, "the correction is still out");
+    assert!(!layer.parts()[0].hidden, "and the base is still in");
+    assert!(layer.enabled, "and the layer itself was never switched off");
+}
+
+/// TRACES: FR-DEV-19a
+/// The base is a part like any other for this: it can be left out, and the
+/// key lands in the mask block since that is where the base is written.
+#[test]
+fn a_hidden_base_is_written_into_the_mask_block() {
+    let mut graph = EditGraph::default_chain();
+    let mut layer = corrected("m1", Join::Union);
+    layer.base_mut().hidden = true;
+    graph.masks_mut().push(layer);
+
+    let restored = round_trip(&graph);
+    let layer = &restored.masks().layers()[0];
+    assert!(layer.parts()[0].hidden, "the base came back hidden");
+    assert!(!layer.parts()[1].hidden, "and only the base");
+}
+
 /// A part carries its own edge, which is the whole reason it is a part rather
 /// than a second source on the layer: a model's soft coverage and a stroke
 /// painted where it stopped short want different boundaries.