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/beat-alternate.sexp

@@ -0,0 +1,21 @@
+;; beat-alternate construct
+;; Alternates between sources on each beat
+;;
+;; Usage in recipe:
+;;   (construct beat-alternate :path "constructs/beat-alternate.sexp")
+;;   (def segments (beat-alternate beats-data (list video-a video-b)))
+
+(define-construct beat-alternate
+  "Alternate between sources on each beat"
+  (analysis sources)
+  ;; Body: map over time pairs, return segment descriptors
+  (let [times (get analysis :times)
+        pairs (zip-pairs (cons 0 times))
+        n-sources (len sources)]
+    (map-indexed
+      (fn [i pair]
+        (dict :source (nth sources (mod i n-sources))
+              :start (first pair)
+              :end (nth pair 1)
+              :effects (list)))
+      pairs)))