Remove unused Polars JSON support

[rob-p]

Summary

Narrow af-anndata's direct Polars requests to the I/O it actually performs.

• disable Polars defaults on the direct dependency
• retain the polars-io facade used by current imports
• retain CSV/TSV support
• remove polars-io JSON support; all JSON handling here uses serde_json

This removes the flate2, simd-json, and zstd feature edges from polars-io for this crate. It does not alter the DataFrame types crossing the anndata API boundary or any conversion semantics.

Companion dependency cleanups: scverse/anndata-rs#43 and COMBINE-lab/grangers#54. Simpleaf's CI/LTO cleanup is COMBINE-lab/simpleaf#206.

Validation

• all 3 af-anndata unit tests pass against registry anndata 0.7.0
• the af-anndata doctest passes
• the same tests pass with the companion anndata narrow-feature branch
• the complete simpleaf dependency graph builds and all 77 simpleaf release-profile tests pass with all companion branches applied