feat(async): AsyncDatasetWrapper query helpers

[jeswr]

What this does

Adds an asynchronous counterpart of DatasetWrapper for datasets whose contents cannot be read synchronously — for example disk- or remote-backed stores:

• AsyncDatasetWrapper (src/async/AsyncDatasetWrapper.ts) implements the proposed RDF/JS AsyncDatasetCore interface by delegation — Promise-based size/add/delete/has, for await iteration, and (potentially lazily evaluated) match views — and offers the same protected query helpers as DatasetWrapper: subjectsOf, objectsOf, instancesOf, matchSubjectsOf, matchObjectsOf, each returning an AsyncIterable<T> of mapping-class instances.
• IAsyncTermWrapperConstructor (src/async/type/IAsyncTermWrapperConstructor.ts) mirrors ITermWrapperConstructor for the asynchronous surface: new (term, dataset: AsyncDatasetCore, factory) => T.
• The constructor accepts an AsyncDatasetCore or any synchronous DatasetCore source (an instance, a promise of one, or a lazy thunk). Synchronous sources are adapted automatically via the @jeswr/async-dataset foundation package (this repo's first runtime dependency — a deliberate, visible decision), so existing sync stores such as an n3 Store work unchanged.
• The @rdfjs/types devDependency moves to jeswr/types#async-eventful-dataset-types, which adds the proposed AsyncDatasetCore/AsyncDataset interfaces (see feat: add AsyncDataset and EventfulDataset interface variants jeswr/types#2 for the upstream type proposal).
• Ports the async_dataset_wrapper.test.ts slice with AsyncParent/AsyncChild/AsyncParentDataset fixture models covering all five helpers, both wrapping paths (already-async and sync-backed, including a lazy thunk), iteration, size, match, and the add/has/delete round trip.

Scope

Supersedes the AsyncDatasetWrapper slice of #73 (dataset-side async concern) — deliberately minus the notification surface (on/off), the default-graph-only Triple restriction, and the datasetFactory constructor parameter from that omnibus branch, which belong to separate slices. The term-side async concern (AsyncTermWrapper and async mappings) is a separate upcoming slice that builds on the IAsyncTermWrapperConstructor type introduced here; the test models in this PR intentionally use plain hand-rolled classes so this slice stands alone.

No issue is closed by this PR.

Checks

• npx tsc, npm test (133 tests, 0 fail; new code at ~99% line / 100% branch coverage), npx typedoc (no new warnings) and npm audit --omit=dev --audit-level=moderate all pass locally on Node 24.

Review timing: This draft was prepared with Claude; I (@jeswr) will personally review it before it progresses. I'm currently batching a lot of work in flight, so expect active review Wednesday-Friday (8-10 July).