feat: GraphScopedDataset — read from many graphs, write to one

[jeswr]

What this does

Adds GraphScopedDataset: a dataset view that reads from a configurable set of graphs and writes to a single graph, generalizing NamedGraphDataset (which is the special case of reading from and writing to the same one graph, and stays untouched for back-compat).

• Reads union the configured readGraphs, projected onto the default graph, with per-triple deduplication (a triple appearing in several read graphs is exposed once). readGraphs === undefined reads every graph — a deduplicated union view.
• Writes (add / delete) are rewritten into the configured writeGraph; delete leaves copies of the triple in other graphs untouched. Non-default-graph input quads throw NamedGraphError (via the existing ensureDefaultGraph).
• has answers from the read scope, consistent with iteration.
• match(s, p, o[, defaultGraph]) mirrors NamedGraphDataset's self-wrapping pattern and throws TermTypeError for non-default graph arguments.
• DatasetWrapper gains a protected scoped(writeGraph, readGraphs, klass) helper beside named(), accepting IRIs or terms; IGraphScopedDatasetConstructor is added to src/type/. Both new symbols are exported from mod.ts.

Because the projection rewrites every read quad to the default graph, existing TermWrapper/DatasetWrapper models work unchanged against quads that live in named graphs — including the common read-many/write-one asymmetry (e.g. read the union of a document's graphs, write changes to one designated graph).

Why

Addresses the graph-scoping follow-up from #43 (@jaxoncreed): LDO-style control over which graph writes land in, independent of which graphs are read.

Relation to #73

Supersedes the GraphScopedDataset read-write graph-scoping slice of the #73 omnibus, including its union_graph / projected_dataset read-write test slices. It deliberately excludes the rest of that omnibus: no events plumbing (NotifyingDatasetCore), no LazyMaterialize, and no breaking default-graph-only DatasetWrapper rework — this is a purely additive change.

Notes

• Triple deduplication keys terms by termType/value (plus language/direction/datatype for literals, recursion for quoted triples), the same keying used in fix: DatasetWrapper helpers yield distinct terms #88; if both land, the helper can be shared in a follow-up.
• Tests: test/unit/graph_scoped.test.ts (explicit read scope + union view) and test/unit/graph_scoped_integration.test.ts (TermWrapper models reading across graphs and writing into the scoped graph). New code is at 100% line coverage; npm test and npx typedoc are green.

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).