Fix await using held = await ablo.<model>.claim(...) failing to typecheck. claim() now returns a HeldClaim<T> — a Claim<T> with data, release, revoke, and the async disposer made Required (they’re optional on the base Claim<T>, which also models observed peer claims that lack them). A held claim is therefore assignable to AsyncDisposable, so the await using auto-release pattern compiles. Observed claim surfaces still return the looser Claim<T>. HeldClaim is exported.
