`HistoricalDecompositionResult.hdi()` computes the HDI of `baseline` and discards it

[thomaspinder]

Severity: P3
Location: src/impulso/results.py:374

Description

Passing the whole Dataset to az.hdi wastes 16–20% of the call; the single-DataArray pattern used by its siblings is bit-identical.

Suggested fix

Pass posterior_predictive["hd"].

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Found by the 2026-07-29 adversarial audit (reports/audit-2026-07-29.md); every finding was independently verified with a reproduction before filing.