`@Observable` + `@unchecked Sendable` on `AsyncMetadata` races more than the comment admits

The class comment acknowledges a benign race on the `__cache` dictionary insert. But `@Observable` rewrites every stored property — including `__cache` — into accessors that touch an `ObservationRegistrar`. Concurrent mutation therefore races the registrar's own internal state, not just the dictionary.

Two concurrent first-lookups producing a wasted `Task` is benign; two concurrent registrar mutations are not the same category of risk.

Note also that `@Observable` is applied only under `#if swift(>=6)`, so the iOS 16 target has different reactivity semantics from the modern one.

Start by locating AsyncMetadata and inspect how @Observable is applied under the Swift 6 conditional, including the iOS 16 path. Compare the concurrent cache lookup and ObservationRegistrar behavior in both configurations; done requires an explicit resolution of the registrar race and the differing reactivity semantics.

Written by the indexing model from the issue text.