Initial commit: video effects processing system

Add S-expression based video effects pipeline with modular effect
definitions, constructs, and recipe files.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

constructs/slice-every-n.sexp

@@ -0,0 +1,53 @@
+;; slice-every-n construct - group every N beats into one segment
+;;
+;; Usage:
+;;   (construct slice-every-n :path "constructs/slice-every-n.sexp")
+;;   (def segments (slice-every-n beats-data 4
+;;                   :init 0
+;;                   :reducer (fn [acc i start end]
+;;                              {:source video-a
+;;                               :effects (list {:effect invert})
+;;                               :acc (inc acc)})))
+;;
+;; Groups every N analysis times into one segment, calling reducer once per group
+
+(define-construct slice-every-n
+  "Group every N analysis beats into segments"
+  (analysis n)
+  ;; 'init' and 'reducer' come from keyword args
+  ;; Reducer receives: (acc, i, start, end) where start/end are audio beat times
+  ;; Reducer returns: {:source src :effects fx :acc new-acc}
+  ;;   Optionally include :start/:end to override (e.g., for wrapping/randomizing)
+  ;;   :duration is calculated from start/end (use :duration to override)
+  ;;   Return :skip true to skip this segment
+  (let [times (get analysis :times)
+        ;; Group times into chunks of n
+        grouped (chunk-every times n)]
+    (nth
+      (reduce
+        (fn [state group]
+          (let [acc (first state)
+                segments (nth state 1)
+                i (len segments)
+                audio-start (first group)
+                audio-end (last group)
+                audio-duration (- audio-end audio-start)
+                ;; Call user's reducer with audio beat times
+                result (reducer acc i audio-start audio-end)
+                new-acc (get result :acc)]
+            ;; Skip if reducer returns :skip true
+            (if (get result :skip false)
+                (list new-acc segments)
+                (let [;; Use reducer's start/end/duration if provided, else use audio times
+                      seg-start (get result :start audio-start)
+                      seg-end (get result :end audio-end)
+                      seg-duration (get result :duration (- seg-end seg-start))
+                      segment (dict :source (get result :source)
+                                   :start seg-start
+                                   :end seg-end
+                                   :duration seg-duration
+                                   :effects (get result :effects))]
+                  (list new-acc (append segments segment))))))
+        (list init (list))
+        grouped)
+      1)))