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/demo.sx

@@ -6,7 +6,7 @@
 ;; below show the shape of queries we want, and the test suite runs
 ;; them against synthetic in-memory tuples loaded via dl-program-data.
 ;;
-;; Six thematic demos:
+;; Seven thematic demos:
 ;;
 ;;   1. Federation — follow graph, transitive reach, mutuals, FOAF.
 ;;   2. Content    — posts, tags, likes, popularity, "for you" feed.
@@ -15,6 +15,7 @@
 ;;                      follow (transitively)" multi-domain query.
 ;;   5. Tag co-occurrence — distinct (T1, T2) pairs with counts.
 ;;   6. Shortest path  — weighted-DAG path enumeration + min agg.
+;;   7. Org chart      — transitive subordinate + headcount per mgr.
 
 ;; ── Demo 1: federation follow graph ─────────────────────────────
 ;; EDB: (follows ACTOR-A ACTOR-B) — A follows B.
@@ -132,6 +133,24 @@
        (is W (+ W1 W2)))
      (shortest X Y W <- (path X Y _) (min W C (path X Y C))))))
 
+;; ── Demo 7: org chart + transitive headcount ───────────────────
+;; Manager graph: each employee has a single manager. Compute the
+;; transitive subordinate set and headcount per manager.
+;;
+;; EDB:
+;;   (manager EMP MGR)            — EMP reports directly to MGR
+;; IDB:
+;;   (subordinate MGR EMP)        — EMP is in MGR's subtree
+;;   (headcount MGR N)            — number of subordinates under MGR
+(define
+  dl-demo-org-rules
+  (quote
+    ((subordinate Mgr Emp <- (manager Emp Mgr))
+     (subordinate Mgr Emp
+       <- (manager Mid Mgr) (subordinate Mid Emp))
+     (headcount Mgr N
+       <- (subordinate Mgr Anyone) (count N E (subordinate Mgr E))))))
+
 ;; ── Loader stub ──────────────────────────────────────────────────
 ;; Wiring to PostgreSQL would replace these helpers with calls into
 ;; rose-ash's internal HTTP RPC (fetch_data → /internal/data/...).