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/echo.sexp

@@ -1,9 +1,10 @@
 ;; Echo effect - motion trails using frame buffer
-;; @param num_echoes int [1, 20] default 4
-;; @param decay float [0, 1] default 0.5
 
 (define-effect echo
-  ((num_echoes 4) (decay 0.5))
+  :params (
+    (num_echoes :type int :default 4 :range [1 20])
+    (decay :type float :default 0.5 :range [0 1])
+  )
   (let* ((buffer (state-get 'buffer (list)))
          (new-buffer (take (cons frame buffer) (+ num_echoes 1))))
     (begin