datalog: shortest-path demo on weighted DAG (176/176)

dl-demo-shortest-path-rules: path enumerates X→Z with cost
W = sum of edge weights via is/+; shortest filters to the
minimum cost path per (X, Y) pair via min aggregation.

3 demo tests cover direct/multi-hop choice, multi-hop wins on
cheaper route, and unreachable-empty.

Note: cycles produce infinite distance values without a depth
filter; the rule docstring flags this and suggests adding
(<, D, MAX) for graphs that may cycle.

lib/datalog/tests/demo.sx

@@ -237,6 +237,31 @@
           (quote (tag-pair-count cooking vegetarian N)))
         (list {:N 2}))
 
+      ;; ── Shortest path on a weighted DAG ──────────────────
+      (dl-demo-test-set! "shortest a→d via DAG"
+        (dl-query
+          (dl-demo-make
+            (quote ((edge a b 5) (edge b c 3) (edge a c 10) (edge c d 2)))
+            dl-demo-shortest-path-rules)
+          (quote (shortest a d W)))
+        (list {:W 10}))
+
+      (dl-demo-test-set! "shortest a→c picks 2-hop"
+        (dl-query
+          (dl-demo-make
+            (quote ((edge a b 5) (edge b c 3) (edge a c 10)))
+            dl-demo-shortest-path-rules)
+          (quote (shortest a c W)))
+        (list {:W 8}))
+
+      (dl-demo-test-set! "shortest unreachable empty"
+        (dl-query
+          (dl-demo-make
+            (quote ((edge a b 5) (edge b c 3)))
+            dl-demo-shortest-path-rules)
+          (quote (shortest a d W)))
+        (list))
+
       (dl-demo-test-set! "no access without grant"
         (dl-query
           (dl-demo-make