datalog: parser + 18 tests + conformance harness (Phase 1 done, 44/44)

Tokens → list of {:head :body} / {:query} clauses. SX symbols for
constants and variables (case-distinguished). not(literal) in body
desugars to {:neg literal}. Nested compounds permitted in arg
position for arithmetic; safety analysis (Phase 3) will gate them.

Conformance harness wraps lib/guest/conformance.sh; produces
lib/datalog/scoreboard.{json,md}.

lib/datalog/conformance.sh

@@ -0,0 +1,3 @@
+#!/usr/bin/env bash
+# Thin wrapper — see lib/guest/conformance.sh and lib/datalog/conformance.conf.
+exec bash "$(dirname "$0")/../guest/conformance.sh" "$(dirname "$0")/conformance.conf" "$@"