events: injected federation transport (fed-sx-ready) + 6 tests

fetch abstracts how a peer's agenda arrives: (fetch peer-id ws we) ->
{:status :ok :occurrences} | {:status :error}. ev/federated-agenda-via merges
local + trusted peers fetched via the transport; unreachable peers degrade
gracefully. ev/peer-fetch = in-process adapter; ev/federation-status reports
reachability. A real fed-sx transport drops in unchanged. 278/278 green.

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

lib/events/federation.sx

@@ -163,3 +163,70 @@
       (some
         (fn (iv) (ev-fed-overlaps? iv qs qe))
         (ev/federated-busy local-db peers trust actor)))))
+
+;; ---- injected transport (real fed-sx / signed fetch) ----
+;; The in-process merge above expands a peer's local :store directly. In
+;; production a peer's agenda arrives over a transport. `fetch` abstracts that:
+;;   (fetch peer-id ws we) -> {:status :ok :occurrences (...)} | {:status :error :reason ...}
+;; The same merge works for any transport; an unreachable peer (:error) is
+;; skipped (graceful degradation), never breaking the agenda.
+
+(define
+  ev-find-peer
+  (fn
+    (peers pid)
+    (cond
+      ((empty? peers) nil)
+      ((= (ev/peer-id (first peers)) pid) (first peers))
+      (else (ev-find-peer (rest peers) pid)))))
+
+;; In-process transport adapter: resolves a peer-id against a peer list and
+;; expands its :store. Lets the in-process model run through the same `fetch`
+;; interface a remote transport implements.
+(define
+  ev/peer-fetch
+  (fn
+    (peers)
+    (fn
+      (pid ws we)
+      (let
+        ((p (ev-find-peer peers pid)))
+        (if
+          (nil? p)
+          {:status :error :reason :unknown-peer}
+          {:status :ok :occurrences (ev/agenda (ev/peer-store p) ws we)})))))
+
+;; Local agenda (:local) merged with each trusted peer's agenda fetched via the
+;; injected `fetch` transport, sorted by start, tagged with :origin. Peers that
+;; fail to fetch contribute nothing.
+(define
+  ev/federated-agenda-via
+  (fn
+    (local-store trusted-ids ws we fetch)
+    (let
+      ((acc (list)))
+      (begin
+        (for-each
+          (fn (o) (append! acc o))
+          (ev-tag-origin (ev/agenda local-store ws we) :local))
+        (for-each
+          (fn
+            (pid)
+            (let
+              ((res (fetch pid ws we)))
+              (when
+                (= (get res :status) :ok)
+                (for-each
+                  (fn (o) (append! acc o))
+                  (ev-tag-origin (get res :occurrences) pid)))))
+          trusted-ids)
+        (ev-fed-sort acc)))))
+
+;; Reachability report: ((peer-id :ok|:error) ...) for the trusted peers.
+(define
+  ev/federation-status
+  (fn
+    (trusted-ids ws we fetch)
+    (map
+      (fn (pid) (list pid (get (fetch pid ws we) :status)))
+      trusted-ids)))