stacked_did: R9 P1 — sample_share parity + DOF asserts on all variants

R9 verdict was ⚠️ still on parameter-coverage P1: sample_share had no
R-parity fixture, and population/anticipation1 tests asserted SE only
(not BM DOF).

Source-tree additions:
- benchmarks/data/stacked_did_sample_share_panel.csv (NEW): pre-stacked
  fixture for weighting='sample_share'.
- benchmarks/R/generate_stacked_did_golden.R: added sample_share variant
  to the variants list; loops over all 4 variants now.
- benchmarks/data/stacked_did_golden.json: regenerated with sample_share.

Test additions (tests/test_methodology_stacked_did.py):
- test_sample_share_weighting_hc1_matches_clubsandwich_cr1s
- test_sample_share_weighting_hc2_bm_matches_clubsandwich_cr2 (+ DOF)
- Extended test_population_weighting_hc2_bm_matches_clubsandwich_cr2:
  added per-event-time + overall BM DOF parity via CI-width inversion.
- Extended test_anticipation1_hc2_bm_matches_clubsandwich_cr2: same DOF
  parity additions.

89 tests pass. All 3 non-default variants now have full R parity:
event-study SE + overall_se + per-event BM DOF + overall BM DOF.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: igerber

benchmarks/R/generate_stacked_did_golden.R

@@ -51,6 +51,11 @@ variants <- list(
     csv = "benchmarks/data/stacked_did_anticipation1_panel.csv",
     ref_period = -2L,  # shifted: e = -1 - anticipation
     out_key = "anticipation1_unit"
+  ),
+  sample_share = list(
+    csv = "benchmarks/data/stacked_did_sample_share_panel.csv",
+    ref_period = -1L,
+    out_key = "sample_share_unit"  # third Q-weight formula
   )
 )
 

benchmarks/data/stacked_did_golden.json

@@ -105,5 +105,37 @@
       "se_cr2_intercept": 0.387454300846914,
       "coef_es": [-0.09202034132318355, -0.1405590947363716, 2.06336107537375, 1.8472286131028, 1.910082975702512]
     }
+  },
+  "sample_share": {
+    "meta": {
+      "event_times_non_ref": [-2, 0, 1, 2],
+      "es_coef_names": ["delta_neg2", "delta_0", "delta_1", "delta_2"],
+      "ref_period": -1,
+      "anticipation": 0
+    },
+    "unit": {
+      "se_cr1_es": [0.1926945586681986, 0.1774065826127712, 0.179551540289758, 0.2122392518497908],
+      "se_cr2_es": [0.1924916107265304, 0.1771398302930658, 0.1787869223969842, 0.211801063322419],
+      "dof_bm_es": [43.616923190028679, 43.616923190028658, 43.616923190028672, 43.616923190028579],
+      "dof_bm_overall": 43.616923190028821,
+      "se_overall_cr1": 0.1513357911957259,
+      "se_overall_cr2": 0.1509672871980841,
+      "es_coef_names": ["delta_neg2", "delta_0", "delta_1", "delta_2"],
+      "se_cr1_intercept": 0.4871327984687643,
+      "se_cr2_intercept": 0.4880622063955147,
+      "coef_es": [0.1108321656167259, 2.20608363527907, 1.950880164652265, 2.024566452331055]
+    },
+    "unit_subexp": {
+      "se_cr1_es": [0.1793915271522487, 0.1770839950451901, 0.1982000403327432, 0.217155905557332],
+      "se_cr2_es": [0.17853000329591, 0.1762727793305481, 0.1973992806659578, 0.2163342225610217],
+      "dof_bm_es": [50.784770788284384, 50.784770788284128, 50.78477078828427, 50.784770788284149],
+      "dof_bm_overall": 50.78477078828643,
+      "se_overall_cr1": 0.1576457481577655,
+      "se_overall_cr2": 0.1570178024670955,
+      "es_coef_names": ["delta_neg2", "delta_0", "delta_1", "delta_2"],
+      "se_cr1_intercept": 0.3600565608113597,
+      "se_cr2_intercept": 0.3569865903344588,
+      "coef_es": [0.1108321656167259, 2.20608363527907, 1.950880164652265, 2.024566452331055]
+    }
   }
 }