datalog: comparison ops require same-type operands

Type-mixed comparisons were silently inconsistent:

  <("hello", 5)  =>  no result, no error  (silent false)
  <(a, 5)        =>  raises "Expected number, got symbol"

Both should fail loudly with a comprehensible message. Added
dl-compare-typeok?: <, <=, >, >= now require both operands to share
a primitive type (both numbers or both strings) and raise a clear
"comparison <op> requires same-type operands" error otherwise.

`!=` is exempted because it's the polymorphic inequality test
built on dl-tuple-equal? — cross-type pairs are legitimately unequal
and the existing semantics for that case match user intuition.

2 new regression tests; conformance 267/267.

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

lib/datalog/builtins.sx

@@ -82,6 +82,20 @@
                     (else (error (str "datalog arith: unknown op " rel)))))))))
         (else (error (str "datalog arith: not a number — " w)))))))
 
+;; Comparable types — both operands must be the same primitive type
+;; (both numbers, both strings). `!=` is the exception: it's defined
+;; for any pair (returns true iff not equal) since dl-tuple-equal?
+;; handles type-mixed comparisons.
+(define
+  dl-compare-typeok?
+  (fn
+    (rel a b)
+    (cond
+      ((= rel "!=") true)
+      ((and (number? a) (number? b)) true)
+      ((and (string? a) (string? b)) true)
+      (else false))))
+
 (define
   dl-eval-compare
   (fn
@@ -98,6 +112,11 @@
               rel
               " has unbound argument; "
               "ensure prior body literal binds the variable")))
+        ((not (dl-compare-typeok? rel a b))
+          (error
+            (str "datalog: comparison " rel " requires same-type "
+                 "operands (both numbers or both strings), got "
+                 a " and " b)))
         (else
           (let
             ((ok (cond ((= rel "<") (< a b)) ((= rel "<=") (<= a b)) ((= rel ">") (> a b)) ((= rel ">=") (>= a b)) ((= rel "!=") (not (dl-tuple-equal? a b))) (else (error (str "datalog: unknown compare " rel))))))