datalog: reject compound terms in fact / rule-head args

Datalog has no function symbols in argument positions, but the
existing dl-add-fact! / dl-add-rule! validators only checked that
literals were ground (no free variables). A compound like `+(1, 2)`
contains no variables, so:

  p(+(1, 2)).
  => stored as the unreduced tuple `(p (+ 1 2))`

  double(*(X, 2)) :- n(X).  n(3).
  => saturates `double((* 3 2))` instead of `double(6)`

Added dl-simple-term? (number / string / symbol) and an
args-simple? walker, used by:

  - dl-add-fact!: all args must be simple terms
  - dl-add-rule!: rule head args must be simple terms (variables
    are symbols, so they pass)

Compounds remain legal in body literals where they encode `is` /
arithmetic / aggregate sub-goals. Error messages name the offending
literal and point the user at the body-only mechanism.

2 new regression tests; conformance 271/271.

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` → **269/269 across 11 suites**
+`bash lib/datalog/conformance.sh` → **271/271 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 +320,16 @@ large graphs.
 
 _Newest first._
 
+- 2026-05-11 — Compound terms in fact-arg / rule-head positions were
+  silently stored as unreduced expressions. `p(+(1, 2)).` resulted
+  in a tuple `(p (+ 1 2))` (dl-ground? sees no free variables, so it
+  passed). `double(*(X, 2)) :- n(X).` saturated to `double((* 3 2))`
+  rather than `double(6)`. Datalog has no function symbols in arg
+  positions — `dl-add-fact!` and `dl-add-rule!` now reject compound
+  args via a new `dl-simple-term?` (number / string / symbol).
+  Compounds remain legal in body literals where they encode `is` /
+  arithmetic / aggregate sub-goals. 2 new regression tests; 271/271.
+
 - 2026-05-11 — Quoted atoms with uppercase-or-underscore-leading
   names were misclassified as variables. `p('Hello World').` ran
   through the tokenizer's `"atom"` branch and through the parser's