datalog: reject body lits with reserved names

Nested `not(not(P))` silently misparsed: outer `not(...)` is
recognised as negation, but the inner `not(banned(X))` was parsed
as a positive call to a relation called `not`. With no `not`
relation present, the inner match was empty, the outer negation
succeeded vacuously, and `vip(X) :- u(X), not(not(banned(X))).`
collapsed to `vip(X) :- u(X).` — a silent double-negation = identity
fallacy.

Fix in `dl-rule-check-safety`: the positive-literal branch and
`dl-process-neg!` both reject any body literal whose relation
name is in `dl-reserved-rel-names`. Error message names the
relation and points the user at stratified negation through an
intermediate relation.

1 regression test; conformance 260/260.

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

lib/datalog/scoreboard.md

@@ -1,13 +1,13 @@
 # datalog scoreboard
 
-**259 / 259 passing** (0 failure(s)).
+**260 / 260 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
 | tokenize | 30 | 30 | ok |
 | parse | 22 | 22 | ok |
 | unify | 29 | 29 | ok |
-| eval | 38 | 38 | ok |
+| eval | 39 | 39 | ok |
 | builtins | 23 | 23 | ok |
 | semi_naive | 8 | 8 | ok |
 | negation | 10 | 10 | ok |