De-duplicate small helpers in substrait.api (extension merge, sort direction, multi-URN lookup, alias)

[nielspardon]

Follow-up cleanup from review of #204 (non-blocking; DRY).

Several small helpers in the new substrait.api layer re-implement logic that already exists elsewhere; consolidating avoids drift:

• expr._merge_extensions_into (src/substrait/expr.py) hand-rolls extension URN/declaration dedup on strict proto equality, whereas substrait.utils.merge_extension_urns / merge_extension_declarations key on anchor/name identity (as builders/plan._merge_extensions does). The two can disagree on when declarations dedupe.
• expr._sort_direction(descending, nulls_last) (src/substrait/expr.py) duplicates frame._SORT_DIRECTIONS (src/substrait/frame.py) — the same (descending, nulls_last) -> SortDirection mapping in two new files.
• functions._multi_urn_helper (src/substrait/functions.py) loops registry.lookup_function per URN; ExtensionRegistry.list_functions_across_urns already resolves matches across every URN in one call, and FunctionEntry.urn recovers the winning extension.
• Expr.alias (src/substrait/expr.py) repeats the referred_expr[0].output_names[0] = name rewrite already in narwhals/expression._alias.

Suggested direction: extract shared helpers (natural home: substrait.utils / builders.extended_expression) and have both call sites use them.

🤖 Filed with AI assistance as a follow-up to #204.