fix: suspend dividends when equity falls below capital floor

[AlexFiliakov]

Problem

The simulation engine pays dividends (30% of after-tax income) whenever gross >= 0, even when the company has nearly zero equity. A firm with $100 in equity and $50 of operating income still pays $7.88 in dividends, accelerating ruin.

This is unrealistic: real boards suspend dividends well before insolvency and redirect all earnings to rebuilding the capital base.

Impact

• Overstates ruin probability across all deductible levels
• Artificially penalises high-deductible strategies (which are more likely to enter distress)
• Biases the optimal deductible downward

Proposed Fix

Add a capital adequacy floor (e.g., 10% of starting assets). When equity falls below this floor, set the effective retention ratio to 1.0 (retain all earnings, pay no dividends):

RUIN_CAPITAL_FLOOR = INITIAL_ASSETS * 0.10

capital_adequate = assets > RUIN_CAPITAL_FLOOR
effective_retention = np.where(capital_adequate, RETENTION_RATIO, 1.0)

delta = np.where(gross >= 0,
                 after_tax * effective_retention,
                 after_tax)

This should be applied in:

• Simulation engine (ergodic_insurance/simulation.py)
• MonteCarloEngine (ergodic_insurance/monte_carlo.py)
• Any notebook simulation loops

Scope

• The floor percentage (10%) could be configurable via ManufacturerConfig or a simulation parameter
• Consider a graduated scale (e.g., linear taper from 100% retention at 5% capital to normal retention at 20% capital)
• Already implemented locally in notebook 12_vol_drag_vs_prem_drag.ipynb as a proof-of-concept

References

• Notebook 12: Volatility Drag vs Premium Drag experiment
• IRC §162 (dividend deductibility limitations under financial distress)