[RFC 0005] Unified JSON Schema for BCF

[andrewherren]

• Same high-level additions at BART: schema_version, platform, num_covariates (Python), covariate_preprocessor rename, rfx_unique_group_ids R move.
• Rename forest keys: prognostic_forest / treatment_forest / variance_forest.
• Parameter field renames: R b_0_samples/b_1_samples → b0_samples/b1_samples; confirm sigma2_leaf_mu_samples, sigma2_leaf_tau_samples, tau_0_samples.
• Confirm sigma2_init naming (R initial_sigma2 → sigma2_init) — overlaps STO-21.
• bart_propensity_model nested envelope written by the same unified writer (carries its own schema_version).