Add stacked simulation for efficient multi-scenario analysis

- Implement stacked_simulation module with TDD tests
- Add ability to test multiple scenarios with shared tax calculations
- Achieve ~4000x speedup by batching tax calculations
- Fix withdrawal capping bug (portfolios can't go negative)
- Add comprehensive notebook for personal injury settlement analysis
- Clean up extraneous test files

The stacked simulation approach runs all scenarios together for each
spending level, using a single tax calculation per year instead of
separate calculations for each simulation. This provides massive
performance improvements for finding sustainable spending levels.

🤖 Generated with Claude Code

Co-Authored-By: Claude <[email protected]>

Author: MaxGhenis