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.md

@@ -1,6 +1,6 @@
 # datalog scoreboard
 
-**263 / 263 passing** (0 failure(s)).
+**264 / 264 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -8,7 +8,7 @@
 | parse | 22 | 22 | ok |
 | unify | 29 | 29 | ok |
 | eval | 39 | 39 | ok |
-| builtins | 23 | 23 | ok |
+| builtins | 24 | 24 | ok |
 | semi_naive | 8 | 8 | ok |
 | negation | 10 | 10 | ok |
 | aggregates | 23 | 23 | ok |