[EPIC] Stabilize mathematical return contracts

[Cubits11]

Purpose

Make core mathematical APIs predictable, typed, and impossible to misunderstand.

Problems

• frechet_upper may return either a scalar float or a tuple containing diagnostics.
• CC coefficient functions return loose dictionaries with mixed values.
• comp.upper() widens typed Literals to plain str.
• Denominator-zero behavior differs across modules.
• ROC-style J and two-world J are both valid but not clearly separated everywhere.

Target

• Scalar functions return scalars.
• Diagnostic functions return typed result objects.
• Composition types are normalized centrally.
• CC undefined behavior is explicit.
• ROC J and two-world J are named separately.

Candidate Fixes

• Add Comp, CompInput, and normalize_comp.
• Split frechet_upper from diagnostic/uncertainty variants.
• Add ComposabilityCoefficients dataclass or TypedDict.
• Add explicit roc_youden_j and two_world_j.
• Decide legacy behavior for metrics.cc_max.

Definition of Done

• Composition normalization helper exists
• frechet_upper(...) -> float is stable
• Diagnostic return path is explicit
• CC denominator policy is tested
• ROC J vs two-world J semantics are documented and tested

Child Issues

• Normalize composition literals
• Split FH scalar and diagnostic return contracts
• Type composability coefficient result
• Add J semantics tests
• Add CC denominator degeneracy tests