iris: one ScrollController, a Scrollable trait, and Pin

Iris's three points on docs/SCROLL.md, in the shape she proposed: a
controller both scrolling widgets *contain*, rather than a protocol
between them. "I don't like adding methods to widget, it seems like we
can structure things better instead."

`Scroll` becomes `ScrollArea`, because it only scrolls a predefined area.
`ScrollController` holds everything that is not a particular widget's
layout -- the position, the pending delta, the travel left each way, the
pin, the DragGesture and the Flinger -- and `Scrollable` is the trait over
it, one required pair of methods with the rest defaulted.

`Widget` loses `scrolls_itself`, `apply_scroll` and `scroll_offset`. They
existed only so a `Scroll` could drive a `LazySpan` it had no business
wrapping; the span owns its own controller now, so the wrapper, the
measure/apply/place dance between two widgets and `amt`'s two meanings all
go with them. The transcript's tree loses a node: `list` is the layout and
the position.

`.scrollable(axis, pin)` replaces `scrollable`/`scrollable_on`/
`scrollable_to_end` -- one mechanism whose arguments had been hidden in
three names. `LazySpan` has an inherent `scrollable()` that shadows it,
since Rust resolves inherent methods before trait ones: the same word at
the call site, and the wrapping version cannot reach the one widget that
must not be wrapped.

`Pin` says which end either way round: `Start`/`End` are content-relative
and `Neg`/`Pos` axis-absolute, so a caller can say "the bottom" and mean
it whichever way the content runs. They differ only for a reversed span,
which is the whole reason both exist.

One behaviour changes: a delta is applied by the next draw rather than
where it arrives, since the layout is the only thing that knows where the
content ends. Nothing on screen differs -- input is followed by a frame --
but `amt` no longer moves between draws, which several tests were reading.
This also closes SCROLL.md's open question about the pin living in two
places.

Verified: cargo test --workspace (all green, including the layer-1
transcript-fixture fling/selection/top-edge tests), clippy --all-targets
clean, fmt clean, `cargo ndk` check of android-app, and
`run-headless.sh phone --phone --replay flick-120hz.touch`, whose
before/after screenshots show the recorded flick carrying the transcript
back from turn 270 to turn 258 on the Vulkan adapter.

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

AGENTS.md

@@ -76,11 +76,12 @@ Module-by-module intent is in `docs/PLAN.md`'s "Backend layout".
     public API** -- Iris, 2026-09-08), working list, decisions log,
     layout/render design, and texture-atlas design, and the client-core
     crate's design, respectively.
-  - `docs/SCROLL.md` — how anything in iris scrolls: `Scroll` owns the
-    position, the gesture and the fling; a child either gets moved or
-    answers `Widget::scrolls_itself` and is handed deltas. Read it before
-    touching `scroll.rs`, `lazy_span.rs`, or anything that pans, flings
-    or lays out a long list.
+  - `docs/SCROLL.md` — how anything in iris scrolls: one
+    `ScrollController` holds the position, the gesture, the fling and the
+    pin, and the two widgets that scroll (`ScrollArea`, `LazySpan`) own
+    one each through the `Scrollable` trait. Read it before touching
+    `scrollable.rs`, `scroll_area.rs`, `lazy_span.rs`, or anything that
+    pans, flings or lays out a long list.
 - `.dev-updater.ron` — what Dev Updater builds here: the server (run as
   `service: Managed(…)`, supervised by Dev Updater's own implementation
   rather than a script kept here) and the APK, in parallel. It points at