Replace polars with arrow for Rust↔Python data bridge

The upstream ggsql crate (v0.3.0) removed polars in favor of a custom
DataFrame built on Arrow RecordBatch. This updates the Python bindings
to match:

Rust side: IPC serialization now uses arrow::ipc::{StreamWriter,StreamReader}
instead of polars IpcWriter/IpcReader. The py_to_df() helper accepts any
object that pyarrow.table() can convert, so custom readers returning polars
DataFrames still work without changes.

Python side: render_altair() calls df.to_arrow() instead of df.to_polars().
Runtime dependency changed from polars to pyarrow.

Public API changes:
- DuckDBReader.execute_sql() returns pyarrow.Table (was polars.DataFrame)
- DuckDBReader.register() accepts pyarrow.Table (polars still works via
  automatic conversion)
- Spec.data()/layer_data()/stat_data() return pyarrow.Table

Author: cpsievert