feat(events): change notifications on DatasetWrapper (on/off)

[jeswr]

Adds change notifications to DatasetWrapper: consumers can now observe every effective mutation of the underlying data with on(listener) / off(listener).

What it does

• DatasetWrapper still accepts any RDF/JS DatasetCore (constructor signature unchanged) and now internally ensures an eventful layer over it (ensureEventfulDatasetCore). The passed dataset remains the single place of storage — mutations write through to it, so external references to it observe all changes.
• New public API: DatasetWrapper.on(listener) and DatasetWrapper.off(listener), plus the exported IDatasetChangeListener / ChangeEvent types. Listeners are called synchronously with ("add" | "delete", quad).
• Only effective changes notify (per Quad.equals): adding an already-contained quad or deleting an absent one emits nothing.
• Because all wrapper-driven mutations flow through DatasetWrapper.add/delete, events fire uniformly for:
  • direct add/delete calls,
  • property assignments through RequiredAs/OptionalAs mappings (delete of the previous quads, then add — assignments do not deduplicate; clearing an optional property emits only deletes),
  • mutations of Sets produced by SetFrom (add/delete/clear),
  • mutations through named graph views created with named(...), which share the same eventful core.
• Subscribing the same listener twice is a no-op; so is detaching a listener that is not subscribed.

Dependency

This introduces the package's first runtime dependency: the event-emitting DatasetCore lives in @jeswr/eventful-dataset rather than being copied in-repo. It is currently pinned as a github: branch dependency while it is staged; it would move to a released npm version (or wherever the group prefers it to live) before this leaves draft. Flagging it explicitly since "dependencies": {} was a visible property of this package.

Relation to #73

Supersedes the events slice of #73 (Omnibus): the DatasetWrapper.on/off wiring, the NotificationsDatasetCore concern, and the dataset events tests. The EventEmitter/notifying-dataset implementation now comes from the foundation package instead of the in-repo src/EventEmitter.ts + src/dataset/NotifyingDatasetCore.ts copies.

Deliberately not included from #73 (left to their own slices): the Triple/default-graph restriction, the datasetFactory constructor parameter, GraphScopedDataset/ProjectedDataset, and all of the async (AsyncDatasetCore) work. No size/match semantics change here.

Tests

• test/unit/dataset_events.test.ts — direct add/delete notifications, effective-change filtering, listener bookkeeping (off, double-subscribe, multiple listeners), and wrapper-driven events for required/optional/typed properties and Set mappings, on the existing Parent/Child fixtures.
• test/unit/dataset_events_examples.test.ts — the JSDoc-style examples as executable tests: observing a SetFrom-backed children set (live views, per-item events) and an optional email field.

npm i && npx tsc && npm test && npx typedoc && npm audit --omit=dev --audit-level=moderate all pass locally (150 tests, 0 fail; typedoc warnings unchanged from main).

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).