fix: DatasetWrapper helpers yield distinct terms

[jeswr]

What this does

matchSubjectsOf and matchObjectsOf — and therefore subjectsOf, objectsOf and instancesOf, which delegate to them — now yield each term at most once, even when it occurs in multiple matching quads. Previously, data such as

<s> <p> <o1> .
<s> <p> <o2> .

made subjectsOf("p") yield <s> twice, though intuition implies distinct values.

How

Each generator tracks yielded terms in a Set<string> keyed on a value-derived string, so deduplication follows Term.equals semantics rather than object identity (RDF/JS terms compare by value, so the Set<Term> sketched in the issue would not catch equal-but-distinct instances). The key incorporates language, base direction and datatype for literals, and recurses through quoted-triple terms, so terms that differ only in those components are still yielded separately.

Tests

New cases in test/unit/dataset_wrapper.test.ts cover:

• instancesOf yielding a subject once despite duplicate rdf:type paths (default graph + named graph)
• subjectsOf / objectsOf yielding a term once across multiple matching quads
• literals differing only in language tag not being collapsed
• quoted-triple subjects deduplicating correctly

Closes #25

Supersedes: nothing — this concern is issue-only and not part of #61, #73 or #74.

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).