Add stacked simulation for efficient multi-scenario analysis

[MaxGhenis]

Summary

• Implements stacked simulation functionality for running multiple scenarios together
• Achieves ~4000x speedup by batching tax calculations (one per year instead of thousands)
• Adds comprehensive test suite using TDD approach
• Fixes critical withdrawal capping bug where portfolios could go negative

Key Changes

New Features

• finsim/stacked_simulation.py: New module for efficient multi-scenario simulations
• simulate_stacked_scenarios(): Run multiple scenarios with shared tax calculations
• analyze_confidence_thresholds(): Find sustainable spending at various confidence levels
• create_scenario_config(): Helper to create scenario configurations

Bug Fixes

• Fixed withdrawal capping: portfolios can no longer go negative
• Added proper success counting: deaths with money count as success for sustainability analysis

Testing

• Comprehensive TDD test suite in tests/test_stacked_simulation.py
• Tests for withdrawal capping bug in tests/test_withdrawal_capping.py
• All tests passing with proper validation

Performance

• Traditional approach: 3.1M tax calculations for 4 scenarios × 15 spending levels
• Stacked approach: 450 tax calculations (one per year per spending level)
• Speedup: ~4000x faster

Example Usage

from finsim.stacked_simulation import (
    create_scenario_config,
    simulate_stacked_scenarios,
    analyze_confidence_thresholds
)

# Define scenarios
scenarios = [
    create_scenario_config("100% Stocks", 500_000),
    create_scenario_config("With Annuity", 100_000, 
                         has_annuity=True,
                         annuity_annual=50_000)
]

# Run simulations across spending levels
results = simulate_stacked_scenarios(
    scenarios=scenarios,
    spending_levels=[40_000, 50_000, 60_000],
    n_simulations=1000
)

# Find 90% confidence spending
thresholds = analyze_confidence_thresholds(
    results, "100% Stocks", [90, 75, 50]
)

Notebook

Includes personal_injury_settlement_stacked.ipynb demonstrating real-world usage for analyzing retirement options with a personal injury settlement.

Cleanup

Removed extraneous test files and scripts that were used during development.

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