gate_diagnostics: treat non-finite min_pscore/match_rate/ESS as missing (audit #252 Finding B)

[dgenio]

Background

Deferred follow-up from the PR #252 fresh-audit (Finding B). The audit fix was reverted in bef07c3 so it could be handled separately with the simulation proof that statistical-evaluation changes require.

Problem

In gate_diagnostics (src/skdr_eval/recommendation.py), the overlap/match-rate and ESS checks coerce their inputs with a NaN-only guard:

min_ps_val = (
    float(min_ps) if min_ps is not None and not np.isnan(float(min_ps)) else None
)

This treats a non-finite ±inf as a real value rather than "missing". Because the overlap and ESS failures only fire on the finite-small side, an +inf can't currently produce a wrong fail, but it is inconsistent with the recommendation/card paths, which route the same diagnostics through the shared _coerce_optional_float helper (which maps NaN and ±inf → None).

Proposed change

Route min_pscore, match_rate, and ESS through _coerce_optional_float in gate_diagnostics, so non-finite values are reported as "not available" consistently with the rest of the pipeline.

Critical caveat — do NOT coerce pareto_k

pareto_k must keep its current NaN-only guard. A non-finite (+inf) Pareto-k is the catastrophic-tail case and must remain a hard fail; only NaN ("not estimated") should be treated as missing. Coercing pareto_k through _coerce_optional_float would silently downgrade a catastrophic result to "not available" — a real verdict regression.

Acceptance criteria

• min_pscore / match_rate / ESS non-finite → reported as missing (value None, no spurious fail).
• pareto_k == +inf → still fail / overall == "fail".
• pareto_k == NaN → missing (value None, not a fail).
• Simulation proof that the change is verdict-neutral on finite inputs (recovers the known ground-truth gate verdicts), per .claude/CLAUDE.md §2 and docs/agent-context/invariants.md.
• make validate passes.

References

• PR feat: gate insufficient_evidence, cover all estimators, extract decision engine #252, revert commit bef07c3
• _coerce_optional_float in src/skdr_eval/reporting.py
• Robustness tests for the desired behavior are preserved in the bde2a73 history if useful as a starting point.