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>

plans/datalog-on-sx.md

@@ -15,7 +15,7 @@ for rose-ash data (e.g. federation graph, content relationships).
 
 ## Status (rolling)
 
-`bash lib/datalog/conformance.sh` → **256/256 across 11 suites**
+`bash lib/datalog/conformance.sh` → **260/260 across 11 suites**
 (tokenize, parse, unify, eval, builtins, semi_naive, negation, aggregates,
 api, magic, demo). Source is ~3100 LOC, tests ~2900 LOC, public API
 documented in `lib/datalog/datalog.sx`.
@@ -320,6 +320,19 @@ large graphs.
 
 _Newest first._
 
+- 2026-05-11 — Eval-semantics bug-hunt: nested `not(not(P))` was
+  silently misinterpreted. Outer-level `not(...)` is parsed as
+  negation, but the inner `not(banned(X))` was parsed as a regular
+  positive literal naming a relation called `not`. Since no `not`
+  relation existed, the inner match was empty and the outer
+  negation succeeded vacuously, making `vip(X) :- u(X), not(not(banned(X))).`
+  equivalent to `vip(X) :- u(X).` (a silent double-negation = identity
+  fallacy). Fix in `dl-rule-check-safety`: both the positive-literal
+  branch and `dl-process-neg!` now flag any body literal whose head
+  is in `dl-reserved-rel-names`. Error message names the relation and
+  points the user at intermediate-relation stratified negation. 1 new
+  regression test; 260/260.
+
 - 2026-05-10 — Bug-hunt round on parser/safety surfaced 7 real
   bugs, each fixed with regression tests:
   - Reserved relation names (`not`, `count`, `<`, `is`, ...) were