datalog: built-ins + body arithmetic + order-aware safety (Phase 4, 106/106)

New lib/datalog/builtins.sx: (< <= > >= = !=) and (is X expr) with
+ - * /. dl-eval-arith recursively evaluates nested compounds.
Safety analysis now walks body left-to-right tracking the bound
set: comparisons require all args bound, is RHS vars must be bound
(LHS becomes bound), = special-cases the var/non-var combos.
db.sx keeps the simple safety check as a forward-reference
fallback; builtins.sx redefines dl-rule-check-safety to the
comprehensive version. eval.sx dispatches built-ins through
dl-eval-builtin instead of erroring. 19 new tests.

lib/datalog/scoreboard.json

@@ -1,13 +1,14 @@
 {
   "lang": "datalog",
-  "total_passed": 87,
+  "total_passed": 106,
   "total_failed": 0,
-  "total": 87,
+  "total": 106,
   "suites": [
     {"name":"tokenize","passed":26,"failed":0,"total":26},
     {"name":"parse","passed":18,"failed":0,"total":18},
     {"name":"unify","passed":28,"failed":0,"total":28},
-    {"name":"eval","passed":15,"failed":0,"total":15}
+    {"name":"eval","passed":15,"failed":0,"total":15},
+    {"name":"builtins","passed":19,"failed":0,"total":19}
   ],
-  "generated": "2026-05-07T23:40:51+00:00"
+  "generated": "2026-05-07T23:50:44+00:00"
 }