Add GeometryReader

[colemancda]

GeometryReader { geometry in … }, giving views access to the space they're offered. This was the deepest of the remaining gaps because it's the one place data has to flow backwards: Swift resolves a tree before Compose lays it out, so the node tree had no size-feedback path at all.

How it works, and why it terminates

The interpreter measures, reports the size through a string callback, and the resulting store update re-evaluates with real numbers — so a GeometryReader settles over two passes (zero on the first, measured on the second). No new bridge entry point: the size rides the existing string callback as "<width>,<height>".

Two properties keep that from becoming an infinite layout loop, and both are deliberate:

1. The reported size comes from the constraints the parent offers, never from the content. If it were content-derived, content sized from the measurement would feed back into it — and since the first pass measures zero, it would deadlock at zero rather than converge.
2. A report equal to the stored size is dropped, so a steady layout reports once and stops.

An unbounded height (inside a scrolling parent) isn't a real offer, so it reports 0 for that axis rather than infinity; give the reader an explicit .frame(height:) there.

Verification

• swift test — 89 total passing. One test walks the two-pass settling end to end (content reads w=0 h=0, then w=320 h=240 after a report). The other drives the size store directly to pin the settling rule — a new size re-evaluates, the same size does not, a later change does again, and malformed reports never do. That second test is the one guarding against a re-evaluation loop.
• Emulator (new "GeometryReader" screen): reports 360 × 200, and bars drawn at size.width / 2 and / 4 measure exactly half and a quarter of the reader — the number is load-bearing, not just displayed. Toggling the frame re-measures in both directions (200 → 320 → 200) and settles each time.

Scope

GeometryProxy exposes size only — frame(in:), safeAreaInsets, and named coordinate spaces aren't modeled, and there's no .coordinateSpace. Worth knowing that a GeometryReader costs an extra evaluation pass whenever its size changes.