Document cross-language semantic parity matrix

[uriahf]

Goal

Record the Stage 2 R/Python semantic parity conclusions after the merged Stage 0 contract and Stage 1 characterization tests.

What this adds

Adds PARITY.md with:

• the shared target semantic model;
• scenario-by-scenario R/Python grouping semantics;
• reference-line scope for ROC, precision-recall, gains, lift, decision curves, interventions avoided, calibration, and performance tables;
• static semantic parity versus representation differences;
• the characterized Python time-dependent divergence where reference ownership can depend on horizon-specific risk equality;
• a recommended future parity direction that keeps reference_group as compatibility terminology rather than the target domain abstraction;
• an explicit statement that rtichoke_viz should remain unchanged until package-level semantics are aligned.

Scope

Documentation-only. No statistical calculations, public APIs, tests, production behavior, generated documentation, or rtichoke_viz are changed.

Paired with uriahf/rtichoke#165.

[github-actions]

PR Preview Action v1.8.1
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2026-08-21 11:13 UTC