Add DGP functions to prep.py for all supported DiD designs

CLAUDE.md

@@ -195,7 +195,12 @@ pytest tests/test_rust_backend.py -v
   - Integrates with HonestDiD for comprehensive sensitivity analysis
 
 - **`diff_diff/prep.py`** - Data preparation utilities:
-  - `generate_did_data` - Create synthetic data with known treatment effect
+  - `generate_did_data` - Create synthetic data with known treatment effect (basic 2x2 DiD)
+  - `generate_staggered_data` - Staggered adoption data for CallawaySantAnna/SunAbraham
+  - `generate_factor_data` - Factor model data for TROP/SyntheticDiD
+  - `generate_ddd_data` - Triple Difference (DDD) design data
+  - `generate_panel_data` - Panel data with optional parallel trends violations
+  - `generate_event_study_data` - Event study data with simultaneous treatment
   - `make_treatment_indicator`, `make_post_indicator` - Create binary indicators
   - `wide_to_long`, `balance_panel` - Panel data reshaping
   - `validate_did_data`, `summarize_did_data` - Data validation and summary

TODO.md

@@ -92,6 +92,21 @@ Enhancements for `honest_did.py`:
 
 ---
 
+## RuntimeWarnings in Linear Algebra Operations
+
+Pre-existing RuntimeWarnings in matrix operations that should be investigated:
+
+- [ ] `linalg.py:162` - "divide by zero", "overflow", "invalid value" in fitted value computation
+  - Occurs during `X @ coefficients` when coefficients contain extreme values
+  - Seen in test_prep.py during treatment effect recovery tests
+- [ ] `triple_diff.py:307,323` - Similar warnings in propensity score computation
+  - Occurs in IPW and DR estimation methods with covariates
+  - Related to logistic regression overflow in edge cases
+
+**Note**: These warnings do not affect correctness of results but should be handled gracefully (e.g., with `np.errstate` context managers or input validation).
+
+---
+
 ## Rust Backend Optimizations
 
 Deferred from PR #58 code review (completed in v2.0.3):

diff_diff/__init__.py

@@ -71,6 +71,11 @@
     balance_panel,
     create_event_time,
     generate_did_data,
+    generate_ddd_data,
+    generate_event_study_data,
+    generate_factor_data,
+    generate_panel_data,
+    generate_staggered_data,
     make_post_indicator,
     make_treatment_indicator,
     rank_control_units,
@@ -190,6 +195,11 @@
     "validate_did_data",
     "summarize_did_data",
     "generate_did_data",
+    "generate_staggered_data",
+    "generate_factor_data",
+    "generate_ddd_data",
+    "generate_panel_data",
+    "generate_event_study_data",
     "create_event_time",
     "aggregate_to_cohorts",
     "rank_control_units",