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.md

@@ -1,6 +1,6 @@
 # datalog scoreboard
 
-**87 / 87 passing** (0 failure(s)).
+**106 / 106 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -8,3 +8,4 @@
 | parse | 18 | 18 | ok |
 | unify | 28 | 28 | ok |
 | eval | 15 | 15 | ok |
+| builtins | 19 | 19 | ok |