feat: robots.txt editor and tester screen

- golem15.seo.robots singleton (UI-SPEC C10): staging callout and the
  served file (P4); the plugin's own extension adds the editor, the
  tester and the AI bots tab, saved to the robots row with history
- field type robotstxt (E4, live lint, size meter; Decode refuses over
  500 KB) and widget golem15-seo-robotstester (E5) on robots_test, which
  tests the unsaved text and switches through grobotstxt
- saving rules that block a noindex address answers robots.blocks_noindex

models/robots/fields.yaml

@@ -0,0 +1,16 @@
+# robots.txt screen (UI-SPEC C10), tab robots.txt: the staging callout and
+# the served file (P4). The plugin's own form extension on the robots
+# controller adds the editor (E4) and the tester (E5) between them, and the
+# AI bots tab.
+fields:
+    staging:
+        type: partial
+        path: staging
+        span: full
+        tab: golem15.seo::lang.robots.tab_file
+    served:
+        label: golem15.seo::lang.robots.served
+        type: partial
+        path: robots_served
+        span: full
+        tab: golem15.seo::lang.robots.tab_file