datalog: anonymous _ vars are unique per occurrence (Phase 5d, 156/156)

(p X _), (p _ Y) — the two _ are now different variables, matching
standard Datalog semantics. Previously both _ symbols were the same
SX symbol, so unification across them gave wrong answers.

Fix in db.sx: dl-rename-anon-term + dl-rename-anon-lit walk a term
or literal and replace each '_' symbol with a fresh _anon<N>.
dl-make-anon-renamer returns a counter-based name generator scoped
per call. dl-rename-anon-rule applies it to head and body of a
rule. dl-add-rule! invokes the renamer before safety check.

eval.sx: dl-query renames anon vars in the goal before search and
filters '_' out of the projection so user-facing results aren't
polluted with internal _anon<N> bindings.

The previous "underscore in head ok" test now correctly rejects
(p X _) :- q(X) as unsafe (the head's fresh anon var has no body
binder). New "underscore in body only" test confirms the safe
case. Two regression tests for rule-level and goal-level
independence.

lib/datalog/scoreboard.md

@@ -1,13 +1,13 @@
 # datalog scoreboard
 
-**153 / 153 passing** (0 failure(s)).
+**156 / 156 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
 | tokenize | 26 | 26 | ok |
 | parse | 18 | 18 | ok |
 | unify | 28 | 28 | ok |
-| eval | 15 | 15 | ok |
+| eval | 18 | 18 | ok |
 | builtins | 19 | 19 | ok |
 | semi_naive | 8 | 8 | ok |
 | negation | 10 | 10 | ok |