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/scoreboard.json

@@ -0,0 +1,11 @@
+{
+  "lang": "datalog",
+  "total_passed": 44,
+  "total_failed": 0,
+  "total": 44,
+  "suites": [
+    {"name":"tokenize","passed":26,"failed":0,"total":26},
+    {"name":"parse","passed":18,"failed":0,"total":18}
+  ],
+  "generated": "2026-05-07T23:30:07+00:00"
+}