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.

plans/datalog-on-sx.md

@@ -280,6 +280,13 @@ large graphs.
 
 _Newest first._
 
+- 2026-05-08 — Demo: weighted-DAG shortest path. `dl-demo-shortest-
+  path-rules` defines `path` over edges with `is W (+ W1 W2)` for
+  cost accumulation and `shortest` via `min` aggregation. 3 demo
+  tests cover direct/multi-hop choice, multi-hop wins on cheaper
+  route, and unreachable-empty. Added `dl-summary db` inspection
+  helper returning `{<rel>: count}` (4 eval tests).
+
 - 2026-05-08 — Phase 5e perf: first-arg index per relation. db gains
   `:facts-index {<rel>: {<first-arg-key>: tuples}}` mirroring the
   existing `:facts-keys` membership index. `dl-add-fact!` populates