"first_null_row" configuration option

README.md

@@ -243,6 +243,9 @@ topdata:
       min: 0
       max: 175
       hex_width: 2
+  row_generation:
+    - namespace: portraits
+      mode: first_null_row
   class_feat_injections:
     global_feats:
       - feat: feat:literate
@@ -295,6 +298,12 @@ topdata:
       value: 0
 ```
 
+`topdata.row_generation` controls where newly generated rows are allocated for
+specific datasets. The default `after_base` mode preserves existing behavior by
+allocating after the final imported base row. `first_null_row` fills base rows
+whose canonical key is empty and whose emitted 2DA columns are all null-like
+(`null`, empty string, or `****`) before extending the table.
+
 `topdata.class_feat_injections` controls generated rows for every
 `classes/feats/*.json` table. `global_feats` injects concrete `feat:*`
 references unless the class already authors that feat; `require_present` and