events: federated free/busy across trusted peers + 10 tests

Peers publish busy intervals per actor (iCal free/busy model — privacy-
preserving, not event details). ev/peer-with-busy, ev/peer-busy;
ev/federated-busy unions local availability-db busy + trusted peers' published
busy (sorted); ev/federated-free? answers cross-instance availability,
half-open, trust-gated (untrusted peers ignored). 219/219 green.

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

lib/events/federation.sx

@@ -9,10 +9,20 @@
 ;; This is the trust-gated stub: peers publish plain schedules and we fold the
 ;; trusted ones into a single sorted agenda. Real transport (fed-sx / signed
 ;; fetch) slots in behind `ev/peer-agenda` without changing the merge.
+;;
+;; Federated FREE/BUSY follows the iCal model: a peer publishes BUSY intervals
+;; for an actor (not event details — privacy-preserving), and we union local +
+;; trusted-peer busy to answer "is this actor free?" across instances.
+
+(define ev/peer (fn (id store) {:id id :busy (list) :store store}))
+
+;; A peer that also publishes free/busy: `busy` is a list of
+;; (actor ((start end) ...)) pairs.
+(define ev/peer-with-busy (fn (id store busy) {:id id :busy busy :store store}))
 
-(define ev/peer (fn (id store) {:id id :store store}))
 (define ev/peer-id (fn (p) (get p :id)))
 (define ev/peer-store (fn (p) (get p :store)))
+(define ev/peer-busy-table (fn (p) (get p :busy)))
 
 (define
   ev-fed-member?
@@ -96,3 +106,60 @@
   (fn
     (agenda origin)
     (filter (fn (o) (= (get o :origin) origin)) agenda)))
+
+;; ---- federated free/busy ----
+
+;; A peer's published busy intervals for `actor` ((start end) ...), or empty.
+(define
+  ev/peer-busy
+  (fn
+    (peer actor)
+    (let
+      ((row (ev-fed-assoc actor (ev/peer-busy-table peer))))
+      (if (nil? row) (list) (first (rest row))))))
+
+(define
+  ev-fed-assoc
+  (fn
+    (k pairs)
+    (cond
+      ((empty? pairs) nil)
+      ((= (first (first pairs)) k) (first pairs))
+      (else (ev-fed-assoc k (rest pairs))))))
+
+;; All busy intervals for `actor` across the LOCAL availability db plus every
+;; TRUSTED peer's published free/busy, merged and sorted by start.
+;; `local-db` is an availability db (see availability.sx ev-build-avail).
+(define
+  ev/federated-busy
+  (fn
+    (local-db peers trust actor)
+    (let
+      ((acc (list)))
+      (begin
+        (for-each (fn (iv) (append! acc iv)) (ev-busy local-db actor))
+        (for-each
+          (fn
+            (peer)
+            (when
+              (ev/trusts? trust (ev/peer-id peer))
+              (for-each
+                (fn (iv) (append! acc iv))
+                (ev/peer-busy peer actor))))
+          peers)
+        (ev-sort-lists acc)))))
+
+;; Half-open overlap of interval (s e) with window [qs, qe).
+(define
+  ev-fed-overlaps?
+  (fn (iv qs qe) (and (< (first iv) qe) (< qs (first (rest iv))))))
+
+;; Is `actor` free across [qs, qe) considering local + trusted-peer busy?
+(define
+  ev/federated-free?
+  (fn
+    (local-db peers trust actor qs qe)
+    (not
+      (some
+        (fn (iv) (ev-fed-overlaps? iv qs qe))
+        (ev/federated-busy local-db peers trust actor)))))