mod: Ext 16 — ActivityPub-shaped decision export, 350/350

mod/decision->activity maps a decision to a moderation verb (remove→Delete,
ban→Block, hide/escalate→Flag, keep→no activity) shaped like an AP activity,
preserving the precise action. mod/decisions->activities batch-exports dropping
keeps. With wire (Ext 14) + fed trust (Phase 4) the federated moderation path is
end-to-end: decide → activity/wire → peer → trust-gate → apply. +17 tests.

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

lib/mod/activity.sx

@@ -0,0 +1,40 @@
+;; lib/mod/activity.sx — export decisions as ActivityPub-shaped events.
+;;
+;; The rose-ash platform propagates cross-domain effects as ActivityPub-shaped
+;; activities. A moderation decision maps to a moderation verb so the rest of the
+;; platform (and federated peers) can act on it: remove→Delete, ban→Block,
+;; hide/escalate→Flag, keep→no activity. The precise mod action is preserved in
+;; :action so a consumer can disambiguate (e.g. hide vs escalate, both Flag).
+
+(define
+  mod/action->verb
+  (fn
+    (action)
+    (cond
+      ((= action "remove") "Delete")
+      ((= action "ban") "Block")
+      ((= action "hide") "Flag")
+      ((= action "escalate") "Flag")
+      (true nil))))
+
+(define
+  mod/decision->activity
+  (fn
+    (d actor)
+    (let
+      ((verb (mod/action->verb (get d :action))))
+      (if (nil? verb) nil {:type verb :action (get d :action) :actor actor :summary (str "moderation/" (get d :action) " via " (get d :rule)) :object (get d :report-id) :rule (get d :rule)}))))
+
+;; map a batch of decisions to activities, dropping the no-op keeps
+(define
+  mod/decisions->activities
+  (fn
+    (decisions actor)
+    (reduce
+      (fn
+        (acc d)
+        (let
+          ((a (mod/decision->activity d actor)))
+          (if (nil? a) acc (append acc (list a)))))
+      (list)
+      decisions)))