feat: add tail risk attribution dashboard

[RichaShiny]

Adds the interactive Streamlit layer and methodology documentation for the paired tail-risk attribution engine already merged on main.

What this adds:

• average and tail-conditioned marginal cascade-exposure contribution by collateral asset
• tail bad-debt attribution
• probability of positive marginal contribution
• secondary-liquidation and endogenous-price-impact contribution views
• scenario-level shock-versus-contribution inspection
• explicit documentation that leave-one-asset-out effects are marginal counterfactual effects, not an additive decomposition
• README methodology, validation coverage, dashboard description, and project-structure updates

This follow-up does not change the attribution engine itself. It exposes and documents the already-tested model.

Please review and merge manually.