mod: Ext 10 — policy what-if / impact analysis, 260/260

mod/decision-diff compares one report's action under two rule sets;
mod/policy-impact batches a set and returns only the reports whose decision flips;
mod/impact-count / mod/impact-report summarize. Lets a mod team measure a policy
change's blast radius before shipping (e.g. removing spam-hide flips r1 hide→keep).
Pure SX over decide-report. +13 tests.

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

lib/mod/whatif.sx

@@ -0,0 +1,56 @@
+;; lib/mod/whatif.sx — policy what-if / impact analysis.
+;;
+;; Before shipping a policy change, a moderation team needs to know which past or
+;; pending reports would decide differently. mod/decision-diff compares one
+;; report's action under two rule sets; mod/policy-impact runs a whole batch and
+;; returns only the reports whose decision flips. Pure SX over decide-report.
+
+(define
+  mod/decision-diff
+  (fn
+    (r reports rules-a rules-b)
+    (let
+      ((a (get (mod/decide-report r reports rules-a) :action))
+        (b (get (mod/decide-report r reports rules-b) :action)))
+      {:after b :changed (if (= a b) false true) :report-id (mod/report-id r) :before a})))
+
+(define
+  mod/policy-impact
+  (fn
+    (reports rules-a rules-b)
+    (reduce
+      (fn
+        (acc r)
+        (let
+          ((d (mod/decision-diff r reports rules-a rules-b)))
+          (if (get d :changed) (append acc (list d)) acc)))
+      (list)
+      reports)))
+
+(define
+  mod/impact-count
+  (fn
+    (reports rules-a rules-b)
+    (len (mod/policy-impact reports rules-a rules-b))))
+
+(define
+  mod/impact-report
+  (fn
+    (reports rules-a rules-b)
+    (let
+      ((changed (mod/policy-impact reports rules-a rules-b)))
+      (if
+        (empty? changed)
+        "No decisions change."
+        (mod/join-with
+          "\n"
+          (map
+            (fn
+              (d)
+              (str
+                (get d :report-id)
+                ": "
+                (get d :before)
+                " → "
+                (get d :after)))
+            changed))))))