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/scoreboard.json

@@ -1,8 +1,8 @@
 {
   "lang": "datalog",
-  "total_passed": 173,
+  "total_passed": 176,
   "total_failed": 0,
-  "total": 173,
+  "total": 176,
   "suites": [
     {"name":"tokenize","passed":26,"failed":0,"total":26},
     {"name":"parse","passed":18,"failed":0,"total":18},
@@ -13,7 +13,7 @@
     {"name":"negation","passed":10,"failed":0,"total":10},
     {"name":"aggregates","passed":16,"failed":0,"total":16},
     {"name":"api","passed":11,"failed":0,"total":11},
-    {"name":"demo","passed":15,"failed":0,"total":15}
+    {"name":"demo","passed":18,"failed":0,"total":18}
   ],
-  "generated": "2026-05-08T09:30:37+00:00"
+  "generated": "2026-05-08T09:35:25+00:00"
 }