docs: Iris's idea for retiring masked_by -- a Stack that names its mask

She asked whether `masked_by` earns its place, since
`.background(x).masked()` looks like the same thing. Measured: for a
square-cornered surface it is (identical to the pixel on the composer at
the phone's size and density), and what the pair cannot express is a
clip that is not a box, which is why the method stands for now.

Her suggestion, in IRIS_TODO.md's "Reconsider": let `Stack` name where
its mask comes from the way `StackSize::Child(n)` already names where
its size comes from, at which point `masked_by` and `Masked::shape` both
go away.

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

docs/IRIS_TODO.md

@@ -964,6 +964,31 @@ do not duplicate it there.
   both ways and keeping the one that is shorter to explain; delete the
   other rather than keeping two ways.
 
+- [ ] **A `Stack` that chooses its mask the way it chooses its size
+  (Iris, 2026-09-08).** She asked whether `masked_by` deserves to exist:
+  "a method that just does 2 separate things you can already easily do
+  does not deserve to exist." For a square-cornered surface it is indeed
+  redundant -- `.background(rect(BAR_FILL)).masked()` was measured
+  against `.masked_by(rect(BAR_FILL))` on the composer at the phone's own
+  size and density and the two are identical to the pixel. What the pair
+  cannot express is a clip that is not a box: `Painter::set_mask` writes
+  a `RectPrimitive::color(Color::NONE)` at the widget's own region, with
+  no radius, so `.background(rect(fill).radius(r)).masked()` draws a
+  rounded panel and then cuts its content square. Both other call sites
+  (`row.rs`'s fence, `tool.rs`'s raw output) are rounded, which is why
+  the method stands for now.
+  Her suggestion for removing it properly: **`Stack` already names where
+  its size comes from (`StackSize::Child(n)`); let it name where its
+  *mask* comes from the same way.** Then `.background(x)` is the one way
+  to put a surface behind something, and clipping to that surface is a
+  property of the stack rather than a second wrapper -- `masked_by` goes,
+  and `Masked::shape` with it. Worth checking while designing it: what a
+  stack with no mask child means (today's behaviour), whether the mask
+  child must also have been *drawn* first (`set_mask_to_widget` requires
+  it, and `Stack` draws in order, so naming child 0 is safe and naming a
+  later one is not), and what happens when the named child is the same
+  one the size comes from.
+
 ## Build (asked for by Iris, 2026-09-06): a density-independent length unit
 
 - [x] **A third length kind beside relative and pixels, so display scales