datalog: dl-magic-rewrite worklist now drains across rule chains (239/239)

Real bug: the worklist used (set! queue (rest queue)) to pop the
head, which left queue bound to a fresh empty list as soon as the
last item was popped. Subsequent (append! queue ...) was a no-op
on the empty list — so when the head's rewrite generated new
(rel, adn) pairs to enqueue, they vanished. Multi-relation
programs (e.g. shortest -> path -> edge, or chained derived
relations) only had their head's rules rewritten; downstream
rules silently dropped.

Fix: use an index-based loop (idx 0 → len queue), with append!
adding to the same list. Items added after the current pointer
are picked up in subsequent iterations.

2 new regression tests:
- 4-level chain (a → r1 → r2 → r3 → r4) under magic returns 2
- shortest-path demo via magic equals dl-query (1 result)

lib/datalog/scoreboard.json

@@ -1,8 +1,8 @@
 {
   "lang": "datalog",
-  "total_passed": 237,
+  "total_passed": 239,
   "total_failed": 0,
-  "total": 237,
+  "total": 239,
   "suites": [
     {"name":"tokenize","passed":26,"failed":0,"total":26},
     {"name":"parse","passed":20,"failed":0,"total":20},
@@ -13,8 +13,8 @@
     {"name":"negation","passed":10,"failed":0,"total":10},
     {"name":"aggregates","passed":20,"failed":0,"total":20},
     {"name":"api","passed":20,"failed":0,"total":20},
-    {"name":"magic","passed":29,"failed":0,"total":29},
+    {"name":"magic","passed":31,"failed":0,"total":31},
     {"name":"demo","passed":21,"failed":0,"total":21}
   ],
-  "generated": "2026-05-08T14:29:08+00:00"
+  "generated": "2026-05-08T14:40:44+00:00"
 }