Support a header-sized detent

[erickreutz]

Problem

We have a sheet with a compact summary header, scrollable detail content, and a footer. The collapsed detent should fit the header while a second detent exposes the full scrollable content.

auto cannot represent this layout because it resolves from the full content height plus the header height. Numeric detents require consumers to:

1. Observe the header with onLayout.
2. Convert its height to a screen fraction.
3. Update detents after mount.
4. Use a guessed initial height or delay presentation to avoid a visible resize.

Fabric synchronous measurement can hide some React layout corrections, but the header must still be committed before it can be measured. TrueSheet already owns the native header layout, so it is better positioned to resolve this detent before initial presentation.

Proposed API

One possible API:

<TrueSheet
  detents={["header", 0.9]}
  initialDetentIndex={0}
  header={<BookingSummary />}
  footer={<SheetActions />}
  scrollable
>
  <BookingDetails />
</TrueSheet>

Exact naming is open, but the new detent would:

• Resolve from the native height of the header slot rather than total content height.
• Include any required fixed footer, safe-area inset, and sheet chrome so those regions are not clipped.
• Be correct when using initialDetentIndex={0}, without a JS measurement pass or initial resize.
• Recalculate when the header layout changes.
• Behave consistently on iOS, Android, and web.

Existing implementation seam

In 3.11.3, both native implementations already track contentHeight and headerHeight independently:

• iOS calculates the auto detent from contentHeight + headerHeight.
• Android calculates the auto detent from contentHeight + headerHeight + contentBottomInset.

A header-sized detent could use the existing measured headerHeight with a separate detent sentinel, while leaving auto behavior unchanged.

This would support a common bottom-sheet pattern: a dynamic collapsed summary above a separately scrollable expanded body, without hard-coded heights or JS layout synchronization.

[lodev09]

#728 added peek detent to support this