datalog: org-chart + transitive headcount demo (219/219)

Adds dl-demo-org-rules: (subordinate Mgr Emp) over a (manager
EMP MGR) graph, and (headcount Mgr N) using count aggregation
grouped by manager. Demonstrates real-world hierarchy queries
(e.g. "everyone reporting up to the CEO") + per-manager rollup.

3 new demo tests: transitive subordinates of CEO (5 entries),
CEO headcount, and direct manager headcount.

lib/datalog/tests/demo.sx

@@ -262,6 +262,42 @@
           (quote (shortest a d W)))
         (list))
 
+      ;; ── Org chart + headcount ─────────────────────────────
+      (dl-demo-test-set! "ceo subordinate transitive"
+        (dl-query
+          (dl-demo-make
+            (quote
+              ((manager ic1 mgr1) (manager ic2 mgr1)
+               (manager mgr1 vp1) (manager ic3 vp1)
+               (manager vp1 ceo)))
+            dl-demo-org-rules)
+          (quote (subordinate ceo X)))
+        (list
+          {:X (quote vp1)} {:X (quote mgr1)} {:X (quote ic1)}
+          {:X (quote ic2)} {:X (quote ic3)}))
+
+      (dl-demo-test-set! "ceo headcount = 5"
+        (dl-query
+          (dl-demo-make
+            (quote
+              ((manager ic1 mgr1) (manager ic2 mgr1)
+               (manager mgr1 vp1) (manager ic3 vp1)
+               (manager vp1 ceo)))
+            dl-demo-org-rules)
+          (quote (headcount ceo N)))
+        (list {:N 5}))
+
+      (dl-demo-test-set! "mgr1 headcount = 2"
+        (dl-query
+          (dl-demo-make
+            (quote
+              ((manager ic1 mgr1) (manager ic2 mgr1)
+               (manager mgr1 vp1) (manager ic3 vp1)
+               (manager vp1 ceo)))
+            dl-demo-org-rules)
+          (quote (headcount mgr1 N)))
+        (list {:N 2}))
+
       (dl-demo-test-set! "no access without grant"
         (dl-query
           (dl-demo-make