types (breaking): narrow TermWrapper's Literal/Quad member surface; remove IRdfJsTerm

[jeswr]

What this does

TermWrapper currently declares every member of every RDF/JS term type, so language, datatype, direction, subject, predicate, object and graph appear to exist on every wrapper — including wrappers of term types that do not have them. As raised in the #61 discussion, these are false types: language is declared to be defined even on a wrapped NamedNode, where it is undefined at runtime.

This PR narrows the declared type surface to the members common to all term types (termType, value, equals):

• Deletes src/type/IRdfJsTerm.ts (internal, never exported from mod.ts).
• Removes the seven Literal/Quad getters from the class body and re-defines them on TermWrapper.prototype via an Object.defineProperty loop. Runtime behaviour is unchanged (same delegation to the wrapped term, same non-enumerable configurable accessors); they simply no longer appear in the declared type or the emitted .d.ts.
• Adds an optional type parameter TermWrapper<T extends Term = Term> that narrows termType to that of the wrapped term (e.g. TermWrapper<Literal> has termType: "Literal").
• Fallout, per the wip/term wrapper type #74 diff: ensureDatatype/ensureListRoot and TermFrom.instance now take TermWrapper; ensure.ts and LiteralAs.ts cast to Literal internally where they read language/datatype.
• Tests: a new test/unit/term_wrapper_term_members.test.ts covers runtime delegation of all seven members (literal, quad, subclass, and absent-member cases) and guards the type surface with @ts-expect-error assertions and a termType narrowing check.

BREAKING

Consumer code that reads a term-type-specific member off a wrapper (e.g. wrapper.language) no longer type-checks. Narrow to the term type that declares it (e.g. (wrapper as unknown as Literal).language), or — once #83 lands — hold the intersection type returned by TermWrapper.from, which exposes these members on the Literal/Quad side of the intersection without casts. Runtime behaviour is unchanged.

Relationship to #74, #83 and #61

• Supersedes the surface-removal slice of wip/term wrapper type #74: the IRdfJsTerm deletion, the prototype-defined getters, and the ensure.ts/TermFrom.ts fallout. The other slice of wip/term wrapper type #74 (the from static factory) is superseded by feat: TermWrapper.from static factory with intersection return type #83; together the two cover all of wip/term wrapper type #74.
• Complements feat: TermWrapper.from static factory with intersection return type #83: from's intersection return type is what makes the narrowed surface ergonomic to consume.
• Addresses the "false types" argument from the Introduce TermWrapper#from and TermWrapper#in #61 review thread.

Given the ongoing type-direction discussion on #61/#74, this is deliberately a draft and should not merge before that discussion resolves.

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).