docs: document validation scope and external term validation

[jeswr]

Recurring issues (#308, and the writer side in #329/#383) stem from the same expectation gap: N3.js validates per-format grammar, not the terms themselves. This adds a short Validation subsection under Compatibility stating what the parser guarantees, what it deliberately does not check (RFC 3987 well-formedness beyond the grammar, unresolved relative IRIs without baseIRI, literal value vs datatype, BCP 47 language tags, and that the writer trusts its input terms), and a compact validating-Transform snippet composing validate-iri, rdf-validate-datatype, and bcp-47 for users who need term validation.

All claims and the snippet were verified against n3@2.1.0 and current main (e.g. <http://example.org/%ZZ> and "abc"^^xsd:integer parse; the snippet rejects both).

Refs #308

cc @jeswr

[Copilot]

Pull request overview

This PR clarifies N3.js’s validation responsibilities by documenting that parsing validates per-format grammar/syntax (not RDF term well-formedness), and that writing serializes provided terms as-is—bridging the expectation gap referenced in #308 and writer-side issues (#329/#383).

Changes:

• Adds a Validation subsection under Compatibility describing what the parser/writer do and do not validate.
• Provides a short stream-based Transform example for post-parse term validation using external validators.
• Links to proposed parser-level opt-in validation modes (PR #634).

Suppressed comments (1)

README.md:514

• Several typos/grammar issues in this paragraph ("snipped"/"occuring"), and termTypes looks like an accidental code identifier (RDF/JS uses termType; here you likely mean "term types"). Also, the snippet requires additional packages but doesn't mention installing them.

The following code snipped shows how to validate that NamedNodes and Literals are validly formed. Depending on your application you may wish to apply further validation: such as ensuring that nested Quad terms are valid in RDF 1.2, and ensuring that `termTypes` are only occuring in the positions that is valid for RDF 1.1 and RDF 1.2.
```JavaScript

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[github-actions]

🎉 This PR is included in version 2.2.12 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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