Benchmarks + docs for ND broadcast encoding

[robinskil]

Part of #276 (Phase 3).

Goal

Validate the win and document the model.

Tasks

• Criterion + end-to-end benchmarks (memory + throughput) on representative netCDF/Zarr datasets, broadcast-heavy and not.
• Document the encoding model (geometry → encoding), the layering, the boundary-reconciliation rules, and operator encoding capabilities.

Acceptance

• Benchmark numbers recorded in the issue/PR.
• Docs added under docs/ describing the encoding pipeline.