feat(dataframe): expose sort and repartition

[andygrove]

Is your feature request related to a problem or challenge?

Two ordering / layout primitives are missing from the DataFrame API:

• sort — no way to order a DataFrame today without dropping to SQL.
• repartition — no way to control parallelism / partitioning of a
  DataFrame.

Describe the solution you'd like

sort. Two ergonomics options worth considering:

1. SQL-string flavour matching filter(String) / proposed
   withColumn: df.sort("a ASC, b DESC NULLS FIRST"). Parsed via
   parse_sql_expr plus an ORDER BY shim, or via the SQL parser's
   parse_order_by. Cheapest to implement; no Java-side model.
2. Typed: a SortExpr Java record (column, ascending, nullsFirst) and
   df.sort(SortExpr... exprs). Discoverable, IDE-friendly.

Suggest starting with (1) for consistency with filter, then layering
(2) on top if/when an Expr builder lands for joins.

repartition. DataFusion's Partitioning enum has three variants:

• RoundRobinBatch(usize) — df.repartitionRoundRobin(n)
• Hash(Vec<Expr>, usize) — df.repartitionHash(int n, String... columns) (column-name flavour to start; expression variant later)
• UnknownPartitioning(usize) — not user-facing.

Tests in DataFrameTransformationsTest (sort round-trip, partition
count assertion via collect_partitioned once that's wired, or via
plan inspection).

Describe alternatives you've considered

ORDER BY / DISTRIBUTE BY via SQL. Works but loses the lazy
DataFrame composition.

Additional context

Each carries a small Java-side design choice (sort-expression shape,
partitioning constructor shape); fine to land them as two separate PRs
under this issue if that's cleaner than one batched PR.