test(rdf): record the canonicalizer gaps against the extracted library

[jdsika]

Record the RDF canonicalizer gaps against the extracted library

Stacked on linkml#3985 (mirrored on the fork as #24).
The first two commits belong to that PR; this one adds a single test file. It
has to be stacked, because the test compares against diffable-rdf and that
dependency arrives in linkml#3985.

This changes no behaviour. It adds one test file and nothing else.

Why

linkml_runtime/utils/rdf_canonicalize.py and diffable_rdf's canonicalizer
are the same code. The module was extracted into a standalone library at the
maintainers' request (linkml#3295), and the two copies have since
diverged.

While integrating the library I compared them property by property. The
divergence turned out to be worth writing down, so this adds a conformance file
that asserts each property against both implementations and marks whichever
side fails it xfail(strict=True). Strict, so the file is a ratchet in both
directions: when a fix lands on either side its case starts passing and the
suite fails until the mark is removed.

Nothing is asserted about the library that is not also asserted about linkml,
so the library is not taken on trust.

What it finds

Nine gaps in linkml's copy. Two are silent data corruption, which is the part
worth a second look — the output parses cleanly and says something the input
never said, so nothing reports it:

• A base IRI ending in # rewrites every term. Relativizing
  http://ex.org/d#a against base http://ex.org/d# gives <#a>, correct per
  RFC 3986, but rdflib's parser resolves a fragment reference by concatenation
  and reads it back as http://ex.org/d##a.
• A shared rdf:List tail is written twice. rdflib's Turtle writer renders
  ( … ) collection syntax per list, so a tail referenced from two lists
  becomes two separate blank nodes. Nine triples in, eleven out.

The rest fail loudly or produce unusable output:

• N-Triples output is written for a graph pyoxigraph already refused, so it
  fails on line 1. The fallback is reached because a term is non-standard,
  and for N-Triples that term is always one N-Triples cannot write either.
• Fallback sorting uses str.splitlines(), which breaks on U+2028, U+2029,
  U+0085, U+000B, U+000C and U+001C–1E. N-Triples permits those raw inside a
  quoted literal, so one statement becomes two lines, the halves sort
  independently, and the document no longer parses.
• Degraded RDF/XML is ordered by rdflib's graph traversal, degraded Turtle with
  unbound namespaces gets ns1/ns2 names allocated in traversal order, and
  json-ld is absent from the format map so it falls through to rdflib. All
  three vary across processes — tested with four PYTHONHASHSEED values.
• Trailing newlines differ by format: RDF/XML gives none, Turtle gives one.
• A Dataset is a Graph subclass, so it passes the type check and is
  flattened, dropping the graph names.

One gap runs the other way

diffable-rdf drops @base on its own rdflib fallback, and linkml does not.
rdflib_dumper.dumps(..., prefix_map={"@base": ...}) hits exactly that path,
because the metamodel's bare status: testing on a uriorcurie slot
serializes to a relative <testing> that forces the fallback.

That is asserted here too, as an xfail on the library side. It is a bug in the
library, it is ours to fix, and it is the reason this PR does not simply
replace linkml's implementation with the library's.

Why not just delegate

I tried. Replacing the default path with diffable_rdf.canonicalize_rdf_graph
fixes all nine gaps and leaves all four RDF generators byte-identical — but it
regresses test_loaders_no_namespace, because of the @base gap above.

Delegation becomes viable once the library carries base_iri. It also wants a
diff_stable parameter, so the flag in linkml#3985 stops running on the
unfixed local pipeline. Both are on the library's side; I'll raise them there.

Until then this PR just states the facts and leaves the code alone, so
maintainers can decide whether to fix them in place, wait for delegation, or
accept them as known.

Related

Follows from: linkml#3295 · Module added by: linkml#3407 ·
Hardened by: linkml#3524, linkml#3703 · Known fallback issue:
linkml#3803 · Nondeterminism leaks: linkml#3516