refactor: de-duplicate small helpers in the DataFrame/Expr API

[nielspardon]

Follow-up cleanup from the review of #204 (DRY, no behavior change). Four small helpers in the substrait.dataframe / substrait.narwhals layer re-implemented logic that already existed elsewhere; this consolidates each onto a single source of truth so the copies cannot drift apart.

Changes

• Extension merge — replace dataframe.expr._merge_extensions_into (which deduped on strict proto equality) with a shared utils.merge_extensions_into that reuses merge_extension_urns / merge_extension_declarations, keying on the same anchor/name identity as builders.plan._merge_extensions. Used by Expr.order_by and Expr.over.
• Sort direction — drop the duplicate (descending, nulls_last) -> SortDirection mapping. A single _SORT_DIRECTIONS + sort_direction() now lives in dataframe.expr and is used by both dataframe.expr and dataframe.frame.
• Multi-URN lookup — add ExtensionRegistry.find_function(name, signature, urns=None), a generalization of lookup_function (one URN) and list_functions_across_urns (all URNs) over an ordered URN subset. _resolve_over_urns now resolves in one registry call and recovers the winning extension via FunctionEntry.urn instead of looping lookup_function per URN.
• Alias — extract the referred_expr[0].output_names[0] rewrite into a shared builders.extended_expression.alias, used by both Expr.alias and the Narwhals expression wrapper.

Also replaces the empty Exception("") in merge_extension_declarations with an informative NotImplementedError naming the unsupported mapping type.

Notes

The extension-merge and multi-URN changes are behavior-preserving: extension anchors are assigned deterministically per URN/overload and declaration names embed the full overload signature, so the identity-based dedup and the previous proto-equality dedup agree on all real inputs, and find_function selects the same winning URN as the previous per-URN loop.

Testing

• pytest: 528 passed, 30 skipped
• ruff check and ruff format --check: clean
• New unit tests for merge_extensions_into (identity dedup, multiple sources) and find_function (single-URN parity with lookup_function, candidate-order priority, no-match, all-URNs).

Closes #208

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