generate-subnet-config: set attestation_committee_count to number of subnets

Each subnet maps to one attestation committee. The expanded config now
automatically sets config.attestation_committee_count = N so clients
receive the correct committee count via --attestation-committee-count
and partition themselves accordingly.

Author: ch4r10t33r

generate-subnet-config.py

@@ -96,6 +96,13 @@ def expand(template: dict, n_subnets: int) -> dict:
     _validate_template(validators)
 
     result = copy.deepcopy(template)
+
+    # attestation_committee_count must equal the number of subnets so that
+    # each client correctly partitions itself into N separate committees.
+    if "config" not in result:
+        result["config"] = {}
+    result["config"]["attestation_committee_count"] = n_subnets
+
     expanded: list[dict] = []
 
     for i in range(n_subnets):
@@ -170,6 +177,7 @@ def main() -> None:
     print(
         f"Generated {output_path}:\n"
         f"  {n_clients} client(s) × {n_subnets} subnet(s) = {n_nodes} nodes\n"
+        f"  config.attestation_committee_count = {n_subnets}\n"
         f"  Each server contributes exactly 1 node per subnet (no intra-server subnet sharing)"
     )