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.

ui/dr-ui/src/gesture_book.rs

@@ -127,6 +127,13 @@ pub const GESTURES: &[Gesture] = &[
         pointer: "Click the eye on its row",
         keys: "",
     },
+    Gesture {
+        title: "Leave one part out of a mask, and put it back",
+        section: "Develop",
+        touch: "Tap the ring on the part's row",
+        pointer: "Click the ring on the part's row",
+        keys: "",
+    },
     Gesture {
         title: "Pick a collection up to rearrange the tree",
         section: "Collections sidebar",