Say what three retained-layout details mean

Reading this back, three things claim something they do not do.

`OnResize::Translate` is returned by `TextView::on_resize` under a
comment weighing anchored glyphs against reshaping ones, but nothing
consumes it: `try_reuse` asks only whether the answer is `Scale`, so a
widget saying `Translate` is redrawn. Say so on the variant, since the
comment beside it reads as a description of behaviour.

`depend_on_size(child, false)` and `depend_on_size(child, true)` are the
difference between a hint, which is context-free, and a size the child
produced by drawing, which carries every pixel axis the child read. That
is the subtlest rule in the file and it was spelled as a bool; give the
two cases their names.

`draw_started` is the record of what has drawn during the pass under
way, and it worked only because `redraw` removes an id before asking
about it -- nothing emptied the set, so it accumulated the id of every
widget ever drawn, including ones long gone. Empty it with the pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

core/src/widget/mod.rs

@@ -21,6 +21,9 @@ pub use widgets::*;
 #[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
 pub enum OnResize {
     Scale,
+    /// Reserved: nothing reads this yet, so a widget saying it is redrawn.
+    /// Keeping an unchanged drawing in a bigger box needs the widget to say
+    /// *where* in that box it should sit, which is the alignment work.
     Translate,
     #[default]
     Redraw,