Fix CI review Round 11: clear bootstrap_distribution for L_max=1

P1: Clear bootstrap_results.bootstrap_distribution to None when
    L_max == 1. The stored DID_M distribution doesn't match the
    DID_1 summary statistics. Per-horizon bootstrap stats are
    accessible via event_study_ses/cis/p_values.
P2: Add bootstrap_distribution is None assertion to the L_max=1
    bootstrap sync regression test.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: igerber

diff_diff/chaisemartin_dhaultfoeuille.py

@@ -1688,6 +1688,10 @@ def fit(
                     if bootstrap_results.event_study_cis and 1 in bootstrap_results.event_study_cis
                     else (np.nan, np.nan)
                 )
+                # Clear the DID_M distribution - it doesn't match the
+                # DID_1 summary statistics. The per-horizon bootstrap
+                # stats are accessible via event_study_ses/cis/p_values.
+                bootstrap_results.bootstrap_distribution = None
                 bootstrap_results.overall_p_value = (
                     bootstrap_results.event_study_p_values[1]
                     if bootstrap_results.event_study_p_values

tests/test_chaisemartin_dhaultfoeuille.py

@@ -1864,7 +1864,9 @@ def test_L_max_1_suppresses_joiner_leaver_decomposition(self):
         assert not df_jl[df_jl["estimand"] == "DID_-"].iloc[0]["available"]
 
     def test_L_max_1_bootstrap_results_overall_synced(self, data, ci_params):
-        """bootstrap_results.overall_* must match event_study horizon 1."""
+        """bootstrap_results.overall_* must match event_study horizon 1,
+        and bootstrap_distribution must be cleared (DID_M distribution
+        doesn't match the DID_1 summary stats)."""
         n_boot = ci_params.bootstrap(99)
         est = ChaisemartinDHaultfoeuille(
             placebo=False, twfe_diagnostic=False, n_bootstrap=n_boot, seed=42
@@ -1881,6 +1883,8 @@ def test_L_max_1_bootstrap_results_overall_synced(self, data, ci_params):
         assert br.overall_se == br.event_study_ses[1]
         assert br.overall_ci == br.event_study_cis[1]
         assert br.overall_p_value == br.event_study_p_values[1]
+        # bootstrap_distribution cleared (was DID_M, not DID_1)
+        assert br.bootstrap_distribution is None
 
     def test_L_max_1_uses_per_group_path(self, data):
         """L_max=1 uses the per-group DID_{g,1} path (same as L_max >= 2