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/api.sx

@@ -1,8 +1,9 @@
 ;; lib/mod/api.sx — report registry + public entry points.
 ;;
 ;; mod/report files a report (assigning a sequential id) into the in-memory
-;; registry; mod/decide resolves an id and runs the policy engine against the
-;; current registry and rule set.
+;; registry; mod/add-evidence accumulates evidence onto a filed report;
+;; mod/decide resolves an id, runs the policy engine against the current registry
+;; and rule set, and commits the decision to the append-only audit log.
 
 (define mod/*reports* (list))
 (define mod/*counter* 0)
@@ -12,7 +13,10 @@
   mod/reset!
   (fn
     ()
-    (begin (set! mod/*reports* (list)) (set! mod/*counter* 0))))
+    (begin
+      (set! mod/*reports* (list))
+      (set! mod/*counter* 0)
+      (mod/audit-reset!))))
 
 (define
   mod/report
@@ -35,10 +39,34 @@
       nil
       mod/*reports*)))
 
+(define
+  mod/add-evidence
+  (fn
+    (id kind val)
+    (let
+      ((r (mod/get-report id)))
+      (if
+        (nil? r)
+        nil
+        (let
+          ((updated (mod/attach-evidence r (mod/mk-evidence kind val))))
+          (begin
+            (set!
+              mod/*reports*
+              (map
+                (fn (x) (if (= (mod/report-id x) id) updated x))
+                mod/*reports*))
+            updated))))))
+
 (define
   mod/decide
   (fn
     (id)
     (let
       ((r (mod/get-report id)))
-      (if (nil? r) nil (mod/decide-report r mod/*reports* mod/*rules*)))))
+      (if
+        (nil? r)
+        nil
+        (let
+          ((d (mod/decide-report r mod/*reports* mod/*rules*)))
+          (begin (mod/log-decision! d (mod/report-evidence r)) d))))))