Equal-width binning for spectral-parameter maps, exported at package level.

Move binning to standalone `r_analysis bin` subcommand that writes .npy
patch files. Remove inline bin_patches from snap pipeline and store
fg_nocmb maps in parquet for rebinning support. Add pyarrow schema
unification for backwards-compatible parquet loading.

Support precomputed true-parameter pixel subsets, .npy patch files from
r_analysis bin, and integer K-means counts — mixed freely per parameter.

Remove empty 04-kmeans, add runners for precomputed true-parameter
clusters and post-clustering binning pipeline.

New bin.md page, -c flag docs in CLI reference, updated snap.md to
point to bin subcommand instead of inline --bin-* flags.

Update r_analysis examples to use parquet snapshots and --parquet-dir
instead of pickle-based --snapshot flag.