Make LazyVStack and LazyHStack actually lazy

[colemancda]

LazyVStack/LazyHStack were typealiases to VStack/HStack — they had the SwiftUI name but built every element eagerly. They're now real lazy containers.

Where the laziness comes from

List takes its data directly, so it can register an item provider. A lazy stack takes a ViewBuilder, so the data is one level down — inside the ForEach. Since buildBlock returns a single child unwrapped, LazyVStack { ForEach(…) } hands the stack the ForEach itself; a new internal _LazyElementProvider conformance exposes its count, per-element identity keys, and on-demand resolution. Elements are keyed by identity exactly as the eager path was, so element @State still survives scrolling and reordering.

Content that isn't a single ForEach (mixed or static) falls back to resolving children eagerly — correct, just not lazy — rather than failing.

The nesting hazard

ScrollView { LazyVStack { … } } is the idiomatic SwiftUI shape, but it maps to a LazyColumn inside a verticalScroll Column, which measures the child with unbounded height and crashes Compose. So when a ScrollView's only child is a lazy stack, the ScrollView now yields scrolling to it.

Verification

• swift test — 4 new tests, 79 total passing. The load-bearing one builds a 10,000-element LazyVStack and asserts a probe counter is 0 after evaluation, then exactly 1 after requesting index 42 — i.e. nothing is built until asked for. Others cover identity keys, the eager fallback, and LazyHStack.
• Emulator (new "Lazy Stacks" screen): a 500-item LazyHStack and a 10,000-row LazyVStack on one screen, each scrolling independently — reached item 40+ and Row 2964 respectively, with no crash from the ScrollView nesting.

Note

LazyVGrid/LazyHGrid already had their own lazy path and are unchanged.