[gurmeherchawla]

Support ScrollPosition, onScrollGeometryChange, and onScrollPhaseChange

Adds Android support for programmatic scrolling with ScrollPosition, makes .scrollPosition(id:anchor:) two-way, adds
ScrollViewProxy anchors, and brings onScrollGeometryChange and onScrollPhaseChange to every scroll container. The
behavior follows iOS 26, which I checked case by case on the simulator.

Related PR: #532

What changes

ScrollPosition

• ScrollPosition was an unavailable stub. It's now a value type with every iOS initializer, the scrollTo(id:anchor:),
  scrollTo(edge:), scrollTo(point:), and scrollTo(x:y:) variants, and edge, point, viewID, viewID(type:),
  and isPositionedByUser.
• .scrollPosition(_:anchor:) scrolls as the binding changes, animated inside withAnimation.
• .scrollPosition(id:anchor:) used to only report the top item. It now also scrolls when the ID changes, and supports
  anchor.

Matching iOS

• With a nil anchor, scrolling to an ID moves the least distance that shows the whole view.
• The user's scrolling writes ScrollPosition(id:) for the first view whose middle is past the anchor, and leaves
  isPositionedByUser false (what iOS 26 does). Views without IDs write nothing.
• point is nil unless both x and y were set.

ScrollViewReader: scrollTo(_:anchor:) honors anchor, and works on the .id/ForEach views of a non-lazy
ScrollView, which it didn't before.

Geometry: ScrollGeometry and .onScrollGeometryChange work in ScrollView, the lazy stacks and grids, and List.

Phase: both .onScrollPhaseChange overloads report:

Action	Phases	Velocity
Appear	idle → idle once	0
Drag	idle → interacting → idle	0
Fling	interacting → decelerating → idle	points per millisecond at decelerating
Animated scroll	idle → animating → idle	0
Unanimated scroll	none	—

Bug fix: the withAnimation marker read by Animation.isInWithAnimation was never cleared, because its coroutine ran
on Dispatchers.Main, which has no frame clock. Every change after the first withAnimation then animated. It's a
separate commit.

How it works

• ScrollPosition.swift adds a ScrollDriver protocol with two implementations:
  • ScrollViewDriver: ScrollState, plus the layout coordinates that .id/ForEach views register as they're placed.
  • LazyScrollDriver: LazyListState/LazyGridState, plus LazyItemCollector.
• ScrollDriverEffects connects a driver to the reader, position, geometry, and phase APIs. Each container replaces its
  own scroll-to-ID and position code with about three lines.
• Bindings and scroll state are observed with snapshotFlow in effects, never read during composition, so updates don't
  recompose the container. On a device, a LazyVStack with .scrollPosition(id:) scrolls with the same jank as one
  without it.
• Containers clear these values for their content, so nested scroll views (such as carousels in rows) don't act on them.

Notes for review

• List.swift: the LazyColumn is wrapped in an environment scope and re-indented. The diff is 9 lines added and 12
  removed with whitespace ignored.
• Lazy containers only measure visible items, so offsets and content sizes in lazy containers are estimates from the
  visible items' average size; they're exact for uniform rows.
• Offsets exclude the safe-area insets iOS includes, and contentInsets is zero.
• SkipLib has no CGVector, so ScrollEffects.swift adds a minimal Kotlin-only one for
  ScrollPhaseChangeContext.velocity.
• Kotlin needs a few annotations that Swift doesn't:
  • An unusedp: parameter on scrollPosition(_:anchor:), because erasure gives both scrollPosition overloads the same
    JVM signature.
  • A Binding<out Hashable?> declaration on scrollPosition(id:), so $position resolves to the ScrollPosition
    overload.
  • suspend declarations on the driver methods, which run Compose scroll calls in effect coroutines.

Skip Pull Request Checklist:

• REQUIRED: I have signed the Contributor Agreement
• REQUIRED: I have tested my change locally with swift test
• OPTIONAL: I have tested my change on an iOS simulator or device
• OPTIONAL: I have tested my change on an Android emulator or device
• REQUIRED: I have checked whether this change requires a corresponding update in the Skip Fuse UI repository (link related PR if applicable)
• OPTIONAL: I have added an example of any UI changes to the Showcase sample app