Emit encoded arrays in the nd scan + declare schema/ordering

[robinskil]

Part of #276 (Phase 1). Depends on the classifier + constructor sub-issues.

Goal

Wire the encoded-array path into the nd scan so batches actually carry REE/Dict columns, and declare it.

Tasks

• Use the classifier + constructor in beacon-nd-array/src/arrow/batch.rs (chunked + ragged paths).
• Declare the encoded column types on the scan's output schema (the FileScanConfig / source schema).
• Set output_ordering for monotonic broadcast coordinates (enables later sort elimination).

Acceptance

• Queries over netCDF/Zarr return correct results with encoded batches end-to-end.
• Scan output schema reflects the encoding; ordering is advertised for monotonic coords.