mod: Phase 1 — report schema + policy engine on Prolog, 31/31

Reports → Prolog facts (report/3, classification/2, report_count/2); ordered
policy rules compile to policy_action/3 clauses, first match wins via
pl-query-one. Decisions carry their proof (matching rule + conditions +
evidence). Spam/abuse keyword classification, repeated-report escalation via
Prolog join+arithmetic, no-rule→keep default. Registry api + conformance harness.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/mod/engine.sx

@@ -0,0 +1,45 @@
+;; lib/mod/engine.sx — decide a report by querying the policy program.
+;;
+;; build-program assembles the report's facts plus the compiled policy clauses;
+;; decide-report runs the Prolog query and returns a decision. A decision is a
+;; proof, not a bare keyword: it carries the matching rule, the conditions it
+;; required, the evidence that satisfied them, and the report count — everything
+;; Phase 2's audit trail needs to persist a "why".
+
+(define
+  mod/find-rule
+  (fn
+    (rules name)
+    (reduce
+      (fn
+        (acc r)
+        (if (nil? acc) (if (= (mod/rule-name r) name) r acc) acc))
+      nil
+      rules)))
+
+(define
+  mod/build-program
+  (fn
+    (r count rules)
+    (str (mod/report-facts r count) "\n" (mod/rules->program rules))))
+
+(define
+  mod/decide-report
+  (fn
+    (r reports rules)
+    (let
+      ((count (mod/report-count (mod/report-about r) reports))
+        (kinds (mod/classify-keywords r))
+        (id (mod/report-id r)))
+      (let
+        ((program (mod/build-program r count rules)))
+        (let
+          ((db (pl-load program)))
+          (let
+            ((sol (pl-query-one db (str "policy_action(" id ", Action, Rule)"))))
+            (if
+              (nil? sol)
+              {:action "keep" :proof {:evidence kinds :conditions (list) :rule "none" :count count} :report-id id :rule "none"}
+              (let
+                ((rname (dict-get sol "Rule")))
+                (let ((rule (mod/find-rule rules rname))) {:action (mod/rule-action rule) :proof {:evidence kinds :conditions (mod/rule-when rule) :rule rname :count count} :report-id id :rule rname})))))))))