Address PR #371 AI review R2: scenario 15 metadata accuracy

P3 fix: scenario 15's `params.n_groups` was 80 (the switcher cohort
allocator input fed into `gen_reversible(n_groups=N_GOLDEN, ...)`)
while the realized panel actually contains 120 groups (80 switchers +
20 never-treated + 20 always-treated, appended by `gen_reversible`'s
default cohort additions at line 64).

Replace with two explicit fields:
- `n_switcher_groups = 80`: the load-bearing DGP allocator input
- `n_realized_groups = 120`: the actual unique-group count in the
  serialized data

The parity test reads the `data` block directly, not `params`, so it
is unaffected by this metadata change. Resolves the misleading-metadata
finding the reviewer flagged on R2.

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

Author: igerber

benchmarks/R/generate_dcdh_dynr_test_values.R

@@ -683,7 +683,17 @@ res15 <- did_multiplegt_dyn(
 )
 scenarios$multi_path_reversible_by_path_placebo <- list(
   data = export_data(d15),
-  params = list(pattern = "multi_path_reversible", n_groups = N_GOLDEN,
+  # n_switcher_groups records the switcher cohort count fed into
+  # gen_reversible's `counts_per_F_g` allocator (80 = sum c(20, 20, 15,
+  # 10, 10, 5)); the realized panel has 120 unique groups after the
+  # default 20 never-treated + 20 always-treated control rows are
+  # appended (gen_reversible defaults at line 64). Recording both fields
+  # avoids the metadata-vs-data mismatch the reviewer flagged on
+  # PR #371 R2: anyone reusing this scenario's metadata sees both the
+  # switcher count (the load-bearing number for the DGP allocation) and
+  # the realized panel size.
+  params = list(pattern = "multi_path_reversible",
+                n_switcher_groups = N_GOLDEN, n_realized_groups = N_GOLDEN + 40L,
                 n_periods = 10, seed = 115, effects = 3, placebo = 2,
                 by_path = 3, ci_level = 95),
   results = extract_dcdh_by_path(res15, n_effects = 3, n_placebos = 2)

benchmarks/data/dcdh_dynr_golden_values.json

@@ -766,7 +766,8 @@
       },
       "params": {
         "pattern": "multi_path_reversible",
-        "n_groups": 80,
+        "n_switcher_groups": 80,
+        "n_realized_groups": 120,
         "n_periods": 10,
         "seed": 115,
         "effects": 3,