Add robustness checks: wild cluster bootstrap and distributed lag

- Insert Robustness Checks section before Hypotheses Summary
- Wild cluster bootstrap (B=999 Rademacher) addresses G=13 small-cluster problem
- Distributed lag model tests 1-year delayed budget effects (2017-2023 sample)
- Update Data & Methods to describe both robustness checks
- Update Results to reference bootstrap and distributed lag findings
- Fix duplicate robustness paragraph that was appended twice

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

requirements.txt

@@ -8,3 +8,5 @@ python-dotenv
 openpyxl
 statsmodels
 scipy
+matplotlib
+seaborn