datalog: rose-ash demo programs (Phase 10 syntactic, 153/153)

New lib/datalog/demo.sx with three Datalog-as-query-language demos
over synthetic rose-ash data:

  Federation: (mutual A B), (reachable A B), (foaf A C) over a
              follows graph.
  Content:    (post-likes P N) via count aggregation, (popular P)
              for likes >= 3, (interesting Me P) joining follows
              + authored + popular.
  Permissions: (in-group A G) over transitive subgroup chains,
              (can-access A R).

10 tests run each program against in-memory EDB tuples loaded via
dl-program-data.

Wiring to PostgreSQL and exposing as a service endpoint (/internal
/datalog) is out of scope for this loop — both would require
edits outside lib/datalog/. Programs above document the EDB shape
a real loader would populate.

lib/datalog/demo.sx

@@ -0,0 +1,75 @@
+;; lib/datalog/demo.sx — example programs over rose-ash-shaped data.
+;;
+;; Phase 10 prototypes Datalog as a rose-ash query language. Wiring
+;; the EDB to actual PostgreSQL is out of scope for this loop (it
+;; would touch service code outside lib/datalog/), but the programs
+;; below show the shape of queries we want, and the test suite runs
+;; them against synthetic in-memory tuples loaded via dl-program-data.
+;;
+;; Three thematic demos:
+;;
+;;   1. Federation — follow graph, transitive reach, mutuals.
+;;   2. Content   — posts, tags, likes, popularity, "for you" feed.
+;;   3. Permissions — group membership and resource access.
+
+;; ── Demo 1: federation follow graph ─────────────────────────────
+;; EDB: (follows ACTOR-A ACTOR-B) — A follows B.
+;; IDB:
+;;   (mutual A B)            — A follows B and B follows A
+;;   (reachable A B)         — transitive follow closure
+;;   (foaf A C)              — friend of a friend (mutual filter)
+(define
+  dl-demo-federation-rules
+  (quote
+    ((mutual A B <- (follows A B) (follows B A))
+     (reachable A B <- (follows A B))
+     (reachable A C <- (follows A B) (reachable B C))
+     (foaf A C <- (follows A B) (follows B C) (!= A C)))))
+
+;; ── Demo 2: content recommendation ──────────────────────────────
+;; EDB:
+;;   (authored ACTOR POST)
+;;   (tagged POST TAG)
+;;   (liked ACTOR POST)
+;; IDB:
+;;   (post-likes POST N)            — count of likes per post
+;;   (popular POST)                 — posts with >= 3 likes
+;;   (tagged-by-mutual ACTOR POST)  — post tagged TOPIC by someone
+;;                                    A's mutuals follow.
+(define
+  dl-demo-content-rules
+  (quote
+    ((post-likes P N <- (authored Author P) (count N L (liked L P)))
+     (popular P <- (authored Author P) (post-likes P N) (>= N 3))
+     (interesting Me P
+       <-
+       (follows Me Buddy)
+       (authored Buddy P)
+       (popular P)))))
+
+;; ── Demo 3: role-based permissions ──────────────────────────────
+;; EDB:
+;;   (member ACTOR GROUP)
+;;   (subgroup CHILD PARENT)
+;;   (allowed GROUP RESOURCE)
+;; IDB:
+;;   (in-group ACTOR GROUP)         — direct or via subgroup chain
+;;   (can-access ACTOR RESOURCE)    — actor inherits group permission
+(define
+  dl-demo-perm-rules
+  (quote
+    ((in-group A G <- (member A G))
+     (in-group A G <- (member A H) (subgroup-trans H G))
+     (subgroup-trans X Y <- (subgroup X Y))
+     (subgroup-trans X Z <- (subgroup X Y) (subgroup-trans Y Z))
+     (can-access A R <- (in-group A G) (allowed G R)))))
+
+;; ── Loader stub ──────────────────────────────────────────────────
+;; Wiring to PostgreSQL would replace these helpers with calls into
+;; rose-ash's internal HTTP RPC (fetch_data → /internal/data/...).
+;; The shape returned by dl-load-from-edb! is the same in either case.
+(define
+  dl-demo-make
+  (fn
+    (facts rules)
+    (dl-program-data facts rules)))