Add unified LOOCV for TROP joint method with Rust acceleration

Implements proper leave-one-out cross-validation for the TROP joint method,
matching the twostep method's approach per the paper's Equation 5. Adds Rust
backend acceleration for parallel LOOCV grid search and bootstrap variance
estimation (5-15x speedup).

Changes:
- rust/src/trop.rs: Add loocv_grid_search_joint(), bootstrap_trop_variance_joint(),
  and supporting joint model fitting functions
- diff_diff/trop.py: Update _fit_joint() to use LOOCV, add _loocv_score_joint()
  Python fallback, integrate Rust acceleration for bootstrap variance
- diff_diff/_backend.py: Export new Rust functions
- docs/methodology/REGISTRY.md: Document unified LOOCV approach for joint method
- tests/: Add comprehensive tests for joint method LOOCV and Rust/Python parity

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: igerber

.gitignore

@@ -80,3 +80,6 @@ scripts/
 # Launch directories (local only)
 launch/
 launch-video/
+
+# Reference implementations (local only)
+trop_avg_ref/

CLAUDE.md

@@ -138,6 +138,9 @@ test bootstrap::tests::test_webb_mean_approx_zero ... ok
   - `TROPResults` - Results with ATT, factors, loadings, unit/time weights
   - `trop()` - Convenience function for quick estimation
   - Three robustness components: factor adjustment, unit weights, time weights
+  - Two estimation methods via `method` parameter:
+    - `"twostep"` (default): Per-observation model fitting (Algorithm 2 of paper)
+    - `"joint"`: Weighted least squares with homogeneous treatment effect (faster)
   - Automatic rank selection via cross-validation, information criterion, or elbow detection
   - Bootstrap and placebo-based variance estimation
 

README.md

@@ -1267,6 +1267,7 @@ trop = TROP(
 
 ```python
 TROP(
+    method='twostep',           # Estimation method: 'twostep' (default) or 'joint'
     lambda_time_grid=None,      # Time decay grid (default: [0, 0.1, 0.5, 1, 2, 5])
     lambda_unit_grid=None,      # Unit distance grid (default: [0, 0.1, 0.5, 1, 2, 5])
     lambda_nn_grid=None,        # Nuclear norm grid (default: [0, 0.01, 0.1, 1, 10])
@@ -1279,6 +1280,10 @@ TROP(
 )
 ```
 
+**Estimation methods:**
+- `'twostep'` (default): Per-observation model fitting following Algorithm 2 of the paper. Computes observation-specific weights and fits a model for each treated observation, then averages the individual treatment effects. More flexible but computationally intensive.
+- `'joint'`: Joint weighted least squares optimization. Estimates a single scalar treatment effect τ along with fixed effects and optional low-rank factor adjustment. Faster but assumes homogeneous treatment effects.
+
 **Convenience function:**
 
 ```python

diff_diff/_backend.py

@@ -23,10 +23,13 @@
         project_simplex as _rust_project_simplex,
         solve_ols as _rust_solve_ols,
         compute_robust_vcov as _rust_compute_robust_vcov,
-        # TROP estimator acceleration
+        # TROP estimator acceleration (twostep method)
         compute_unit_distance_matrix as _rust_unit_distance_matrix,
         loocv_grid_search as _rust_loocv_grid_search,
         bootstrap_trop_variance as _rust_bootstrap_trop_variance,
+        # TROP estimator acceleration (joint method)
+        loocv_grid_search_joint as _rust_loocv_grid_search_joint,
+        bootstrap_trop_variance_joint as _rust_bootstrap_trop_variance_joint,
     )
     _rust_available = True
 except ImportError:
@@ -36,10 +39,13 @@
     _rust_project_simplex = None
     _rust_solve_ols = None
     _rust_compute_robust_vcov = None
-    # TROP estimator acceleration
+    # TROP estimator acceleration (twostep method)
     _rust_unit_distance_matrix = None
     _rust_loocv_grid_search = None
     _rust_bootstrap_trop_variance = None
+    # TROP estimator acceleration (joint method)
+    _rust_loocv_grid_search_joint = None
+    _rust_bootstrap_trop_variance_joint = None
 
 # Determine final backend based on environment variable and availability
 if _backend_env == 'python':
@@ -50,10 +56,13 @@
     _rust_project_simplex = None
     _rust_solve_ols = None
     _rust_compute_robust_vcov = None
-    # TROP estimator acceleration
+    # TROP estimator acceleration (twostep method)
     _rust_unit_distance_matrix = None
     _rust_loocv_grid_search = None
     _rust_bootstrap_trop_variance = None
+    # TROP estimator acceleration (joint method)
+    _rust_loocv_grid_search_joint = None
+    _rust_bootstrap_trop_variance_joint = None
 elif _backend_env == 'rust':
     # Force Rust mode - fail if not available
     if not _rust_available:
@@ -73,8 +82,11 @@
     '_rust_project_simplex',
     '_rust_solve_ols',
     '_rust_compute_robust_vcov',
-    # TROP estimator acceleration
+    # TROP estimator acceleration (twostep method)
     '_rust_unit_distance_matrix',
     '_rust_loocv_grid_search',
     '_rust_bootstrap_trop_variance',
+    # TROP estimator acceleration (joint method)
+    '_rust_loocv_grid_search_joint',
+    '_rust_bootstrap_trop_variance_joint',
 ]