Add generic streaming interpreter with configurable sources/audio

- Add stream_sexp_generic.py: fully generic sexp interpreter
- Add streaming primitives for video sources and audio analysis
- Add config system for external sources and audio files
- Add templates for reusable scans and macros
- Fix video/audio stream mapping in file output
- Add dynamic source cycling based on sources array length
- Remove old Python effect files (migrated to sexp)
- Update sexp effects to use namespaced primitives

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

sexp_effects/effects/threshold.sexp

@@ -1,14 +1,9 @@
 ;; Threshold effect - converts to black and white
+(require-primitives "color_ops")
 
 (define-effect threshold
   :params (
     (level :type int :default 128 :range [0 255])
     (invert :type bool :default false)
   )
-  (map-pixels frame
-    (lambda (x y c)
-      (let* ((lum (luminance c))
-             (above (if invert (< lum level) (> lum level))))
-        (if above
-            (rgb 255 255 255)
-            (rgb 0 0 0))))))
+  (color_ops:threshold frame level invert))