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/policy.sx

@@ -0,0 +1,71 @@
+;; lib/mod/policy.sx — moderation rules → Prolog clauses.
+;;
+;; A rule is {:name :action :when}. :when is a list of condition forms; each
+;; compiles to a Prolog goal. Rule order is precedence: the engine queries with
+;; pl-query-one, so the first clause that proves wins. The final default rule has
+;; an empty body (true) so every report yields at least :keep — "no rule matched"
+;; is a real result, not a query failure.
+
+(define mod/mk-rule (fn (name action conds) {:when conds :name name :action action}))
+
+(define mod/rule-name (fn (r) (get r :name)))
+(define mod/rule-action (fn (r) (get r :action)))
+(define mod/rule-when (fn (r) (get r :when)))
+
+(define
+  mod/default-rules
+  (list
+    (mod/mk-rule "spam-hide" :hide (list (list :classification "spam")))
+    (mod/mk-rule
+      "abuse-remove"
+      :remove (list (list :classification "abuse")))
+    (mod/mk-rule
+      "repeated-escalate"
+      :escalate (list (list :count-at-least 3)))
+    (mod/mk-rule "default-keep" :keep (list))))
+
+;; ── condition → Prolog goal ──
+;;
+;; (:classification "spam") → classification(Id, spam)
+;; (:count-at-least 3)      → report(Id, B, S), report_count(S, N), N >= 3
+
+(define
+  mod/cond->goal
+  (fn
+    (c)
+    (let
+      ((tag (first c)))
+      (cond
+        ((= tag :classification)
+          (str "classification(Id, " (nth c 1) ")"))
+        ((= tag :count-at-least)
+          (str
+            "report(Id, B, S), report_count(S, N), N >= "
+            (nth c 1)))
+        (true "true")))))
+
+(define
+  mod/conds->body
+  (fn
+    (conds)
+    (if
+      (empty? conds)
+      "true"
+      (mod/join-with ", " (map mod/cond->goal conds)))))
+
+(define
+  mod/rule->clause
+  (fn
+    (r)
+    (str
+      "policy_action(Id, "
+      (mod/rule-action r)
+      ", '"
+      (mod/rule-name r)
+      "') :- "
+      (mod/conds->body (mod/rule-when r))
+      ".")))
+
+(define
+  mod/rules->program
+  (fn (rules) (mod/join-with "\n" (map mod/rule->clause rules))))