mod: Phase 2 — evidence accumulation + proof trees + audit log, 60/60

Reports carry an :evidence list, asserted as evidence/3 facts; reviewer-remove
rule (highest precedence) lets human review override classification. Proof tree
built constructively by re-querying each rule body goal against the same DB with
the report id bound, so derivations carry real unification bindings. Append-only
audit log records decision + proof + evidence snapshot per decide, monotonic seq,
never mutates prior entries. +29 audit tests.

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

lib/mod/policy.sx

@@ -5,6 +5,10 @@
 ;; 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.
+;;
+;; cond->goal takes an id-term so the same condition can be compiled with the
+;; head variable "Id" (for clause bodies) or a concrete report id (for proof-tree
+;; goal-by-goal re-querying in the engine).
 
 (define mod/mk-rule (fn (name action conds) {:when conds :name name :action action}))
 
@@ -15,6 +19,9 @@
 (define
   mod/default-rules
   (list
+    (mod/mk-rule
+      "reviewer-remove"
+      :remove (list (list :evidence "confirmed-abuse")))
     (mod/mk-rule "spam-hide" :hide (list (list :classification "spam")))
     (mod/mk-rule
       "abuse-remove"
@@ -27,31 +34,41 @@
 ;; ── condition → Prolog goal ──
 ;;
 ;; (:classification "spam") → classification(Id, spam)
+;; (:evidence "kind")       → evidence(Id, 'kind', _)
 ;; (:count-at-least 3)      → report(Id, B, S), report_count(S, N), N >= 3
 
 (define
   mod/cond->goal
   (fn
-    (c)
+    (c idterm)
     (let
       ((tag (first c)))
       (cond
         ((= tag :classification)
-          (str "classification(Id, " (nth c 1) ")"))
+          (str "classification(" idterm ", " (nth c 1) ")"))
+        ((= tag :evidence)
+          (str
+            "evidence("
+            idterm
+            ", "
+            (mod/pl-quote (nth c 1))
+            ", _)"))
         ((= tag :count-at-least)
           (str
-            "report(Id, B, S), report_count(S, N), N >= "
+            "report("
+            idterm
+            ", B, S), report_count(S, N), N >= "
             (nth c 1)))
         (true "true")))))
 
 (define
   mod/conds->body
   (fn
-    (conds)
+    (conds idterm)
     (if
       (empty? conds)
       "true"
-      (mod/join-with ", " (map mod/cond->goal conds)))))
+      (mod/join-with ", " (map (fn (c) (mod/cond->goal c idterm)) conds)))))
 
 (define
   mod/rule->clause
@@ -63,7 +80,7 @@
       ", '"
       (mod/rule-name r)
       "') :- "
-      (mod/conds->body (mod/rule-when r))
+      (mod/conds->body (mod/rule-when r) "Id")
       ".")))
 
 (define