fix: use numeric batch index as custom_id (64 char limit, alphanumeric only)

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

control-pipeline/services/decomposition_pass.py

@@ -3489,10 +3489,10 @@ class DecompositionPass:
                     source_ref=ctrl["source_ref"],
                 )
 
-            # Control IDs in custom_id for result mapping
-            ids_str = "+".join(c["control_id"] for c in batch)
+            # Numeric index as custom_id (max 64 chars, alphanumeric only)
+            batch_idx = i // batch_size
             requests.append({
-                "custom_id": f"p0a_{ids_str}",
+                "custom_id": f"p0a_b{batch_idx:05d}",
                 "params": {
                     "model": ANTHROPIC_MODEL,
                     "max_tokens": max(8192, len(batch) * 2000),