Add composable ASCII art with per-cell effects and explicit effect loading

Implements ascii_fx_zone effect that allows applying arbitrary sexp effects
to each character cell via cell_effect lambdas. Each cell is rendered as a
small image that effects can operate on.

Key changes:
- New ascii_fx_zone effect with cell_effect parameter for per-cell transforms
- Zone context (row, col, lum, sat, hue, etc.) available in cell_effect lambdas
- Effects are now loaded explicitly from recipe declarations, not auto-loaded
- Added effects_registry to plan for explicit effect dependency tracking
- Updated effect definition syntax across all sexp effects
- New run_staged.py for executing staged recipes
- Example recipes demonstrating alternating rotation and blur/rgb_split patterns

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

sexp_effects/effects/blend.sexp

@@ -8,7 +8,13 @@
 ;;   pad-color - color for padding in fit mode [r g b]
 
 (define-effect blend
-  ((mode "overlay") (opacity 0.5) (resize-mode "fit") (priority "width") (pad-color (list 0 0 0)))
+  :params (
+    (mode :type string :default "overlay")
+    (opacity :type float :default 0.5)
+    (priority :type string :default "width")
+    (list :type string :default 0 0 0)
+  )
+  )
   (let [a frame-a
         a-w (width a)
         a-h (height a)
@@ -45,4 +51,4 @@
               b-resized))]
     (if (= mode "alpha")
       (blend-images a b opacity)
-      (blend-images a (blend-mode a b mode) opacity))))
+      (blend-images a (blend-mode a b mode) opacity)))