datalog: arith / by zero raises instead of returning inf

`is(R, /(X, 0))` was silently producing IEEE infinity:

  (dl-eval "p(10). q(R) :- p(X), is(R, /(X, 0))." "?- q(R).")
  => ({:R inf})

That value then flowed through comparisons (anything < inf, anything
> inf) and aggregations (sum of inf, max of inf) producing nonsense
results downstream. `dl-eval-arith` now checks the divisor before
the host `/` and raises "division by zero in <expr>" — surfacing
the bug at its source rather than letting infinity propagate.

1 new test; conformance 264/264.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/datalog/scoreboard.json

@@ -1,14 +1,14 @@
 {
   "lang": "datalog",
-  "total_passed": 263,
+  "total_passed": 264,
   "total_failed": 0,
-  "total": 263,
+  "total": 264,
   "suites": [
     {"name":"tokenize","passed":30,"failed":0,"total":30},
     {"name":"parse","passed":22,"failed":0,"total":22},
     {"name":"unify","passed":29,"failed":0,"total":29},
     {"name":"eval","passed":39,"failed":0,"total":39},
-    {"name":"builtins","passed":23,"failed":0,"total":23},
+    {"name":"builtins","passed":24,"failed":0,"total":24},
     {"name":"semi_naive","passed":8,"failed":0,"total":8},
     {"name":"negation","passed":10,"failed":0,"total":10},
     {"name":"aggregates","passed":23,"failed":0,"total":23},
@@ -16,5 +16,5 @@
     {"name":"magic","passed":36,"failed":0,"total":36},
     {"name":"demo","passed":21,"failed":0,"total":21}
   ],
-  "generated": "2026-05-11T07:56:45+00:00"
+  "generated": "2026-05-11T07:59:10+00:00"
 }