docs: Iris's second 2026-09-08 phone report, and the workaround list closed

RUST.md gets the report verbatim with what each of the four defects
actually was, the tests that pin them, and two traps worth not
re-finding (a fixed-coordinate tap that "failed" by 544px because it had
toggled a tool group, and a layer-1 repro that only reproduces inside a
`List`). IRIS.md and DECISIONS.md get the design half: one `Flinger`
whose seam puts the sign convention and the content's end with the
caller, a cancel as a first-class end to a gesture, and why a row is
drawn twice on the frame its height changes.

LAYOUT.md gains the two rules those turned on, since both govern the
layout rather than this pass: padding works in any container and is an
inset or an outset depending on how tight the parent's region is (Iris's
own words), and a widget offered a box it does not fit is drawn again at
its true box in the same frame rather than the next one.

IRIS_TODO.md's "worked around in tool.rs rather than fixed here" is gone
-- Iris, 2026-09-08: "There should never be workaround code." Two of the
four entries are ticked; the two that remain are missing capabilities
rather than defects being dodged, and each now carries a diagnosis of
what building it costs instead of a workaround: an overflow ellipsis
needs `TextBuffer` to have a displayed string distinct from its source
(parley has none of its own, and every byte-offset consumer -- spans,
`byte_at`, `Selection`, `apply_delta` -- moves if the buffer is
truncated), and selectable tool-card text needs a register/unregister
lifecycle across the three routes that rebuild a card, which is where a
stale `Selection` handle panics.

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

docs/LAYOUT.md

@@ -1074,3 +1074,43 @@ places the code is narrower than the design above, each deliberate:
   `iris_core::SHAPE_SHADER` *by name* and runs them in a compute pass,
   so the thing under test is the shader itself rather than a copy of it
   that would be edited alongside.
+
+## What a widget's *offered* box may and may not be (2026-09-08)
+
+Two rules that were each true in one place and missing from a sibling,
+found together by Iris's 2026-09-08 phone report.
+
+**Padding works in whatever container it is placed in, and is an inset or
+an outset depending on how tight that container's region is.** Iris's
+own words, 2026-09-08: "padding should work no matter what container a
+widget is placed in, and acts as both inset and outset depending on how
+tight the parent region is." `Pad` offers its child the region it was
+handed, inset on each side, and reports `used + padding` — so given a
+generous box it insets the child inside it, and given a box already the
+size of the content it reports a larger size and the parent grows. What
+this rules out is any container that offers a padded child a box and then
+ignores what it reported, and any caller that reshapes its tree to avoid
+a `Pad` (which `transcript-ui/src/tool.rs` did until 2026-09-08, at the
+cost of a tool group's 4dp inset).
+
+**A widget offered a box it does not fit is drawn again at the box its
+own reported size implies, in the same frame.** Not next frame. The
+temptation to defer is real — `List::place` offers a row its *cached*
+height precisely so that an unchanged row hits `draw_inner`'s cheap
+skip-or-move path, and `Scroll` sizes its child region from last frame's
+content length for the same reason. But a `Rect` fills whatever region it
+is given (`Size::REST`, and `rect.rs`'s `is_size_independent` doc says
+why it must), and `.background(rect(..))` is the ordinary way to style
+anything — so a one-frame-stale box is a background drawn at the wrong
+size while the text inside it is already right. On screen that is a tool
+card that looks closed while its text is there and open while it is not.
+A `reposition` is not the fix and cannot be: it writes an offset, never a
+size.
+
+The cost is bounded and worth stating, because it is what makes the rule
+safe to apply everywhere: the second draw happens only on the frame a
+widget's own size actually changes, which is a frame that was already
+redrawing it. A widget whose reported size is a function of the box it
+was *offered* would disagree every frame and redraw every frame — which
+is why `List` requires content-sized rows, and has since long before
+this.