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-on.sexp

@@ -0,0 +1,45 @@
+;; slice-on construct - iterate over analysis times with user function
+;;
+;; Usage:
+;;   (construct slice-on :path "constructs/slice-on.sexp")
+;;   (def segments (slice-on beats-data
+;;                   :init 0
+;;                   :reducer (fn [acc i start end]
+;;                              {:source (nth (list video-a video-b) (mod acc 2))
+;;                               :effects (list)
+;;                               :acc (inc acc)})))
+;;
+;; The construct receives:
+;;   - First positional arg as 'analysis' (the analysis data with :times)
+;;   - :init as 'init' (initial accumulator value)
+;;   - :reducer as 'reducer' (the reducer lambda)
+
+(define-construct slice-on
+  "Iterate over analysis times, calling reducer for each slice"
+  (analysis)
+  ;; 'init' and 'reducer' come from keyword args
+  ;; Get times from analysis data
+  (let [times (get analysis :times)
+        pairs (zip-pairs (cons 0 times))]
+    ;; Use nth to get second element of reduce result (the segments list)
+    (nth
+      (reduce
+        (fn [state pair]
+          (let [acc (first state)
+                segments (nth state 1)
+                i (len segments)
+                start (first pair)
+                end (nth pair 1)
+                ;; Call user's reducer function
+                result (reducer acc i start end)
+                ;; Extract new acc and build segment
+                new-acc (get result :acc)
+                segment (dict :source (get result :source)
+                             :start start
+                             :end end
+                             :duration (- end start)
+                             :effects (get result :effects))]
+            (list new-acc (append segments segment))))
+        (list init (list))
+        pairs)
+      1)))