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/scoreboard.md

@@ -1,6 +1,6 @@
 # datalog scoreboard
 
-**143 / 143 passing** (0 failure(s)).
+**153 / 153 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -13,3 +13,4 @@
 | negation | 10 | 10 | ok |
 | aggregates | 10 | 10 | ok |
 | api | 9 | 9 | ok |
+| demo | 10 | 10 | ok |