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/scoreboard.md

@@ -1,6 +1,6 @@
 # datalog scoreboard
 
-**265 / 265 passing** (0 failure(s)).
+**267 / 267 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -8,7 +8,7 @@
 | parse | 22 | 22 | ok |
 | unify | 29 | 29 | ok |
 | eval | 40 | 40 | ok |
-| builtins | 24 | 24 | ok |
+| builtins | 26 | 26 | ok |
 | semi_naive | 8 | 8 | ok |
 | negation | 10 | 10 | ok |
 | aggregates | 23 | 23 | ok |