`narwhals_example.py` fails on main: narwhals layer passes a YAML filename where a URN is expected

[nielspardon]

examples/narwhals_example.py fails on main. The narwhals expression layer still passes the legacy extension filename where the registry now expects an extension URN, so any narwhals expression that resolves a function raises Unknown function.

Reproduction

uv run examples/narwhals_example.py

Exception: Unknown function abs for [i64 {
  nullability: NULLABILITY_REQUIRED
}
]

Triggered by examples/narwhals_example.py:33 — nw.col("id").abs().

Root cause

src/substrait/narwhals/expression.py:20 passes a YAML filename as the URN:

self.expr = scalar_function(
    "functions_arithmetic.yaml", "abs", expressions=[self.expr]
)

but the registry keys on URNs:

>>> r.has_urn("functions_arithmetic.yaml")
False
>>> r.has_urn("extension:io.substrait:functions_arithmetic")
True

The overload itself is fine — lookup_function("extension:io.substrait:functions_arithmetic", "abs", [i64]) resolves abs:i64 — so this is purely the URN argument. It is the only remaining *.yaml literal in src/substrait/narwhals/, so the narwhals layer appears to have been missed when the registry moved to URNs.

Why CI does not catch it

.github/workflows/example.yml runs only builder_example.py, duckdb_example.py, adbc_example.py and pyarrow_example.py. narwhals_example.py and dataframe_example.py are outside that matrix — which CONTRIBUTING.md notes means they have to be run by hand. Of the two, dataframe_example.py currently passes and narwhals_example.py does not. Adding both to the matrix would stop this regressing silently.

Affected versions

Measured on main at 4c0aebb (substrait.version.substrait_version → 0.99.0). Pre-existing, not a recent regression.