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/rgb_split.sexp

@@ -1,9 +1,10 @@
 ;; RGB Split effect - chromatic aberration
-;; @param offset_x float [-50, 50] default 10
-;; @param offset_y float [-50, 50] default 0
 
 (define-effect rgb_split
-  ((offset_x 10) (offset_y 0))
+  :params (
+    (offset_x :type int :default 10 :range [-50 50])
+    (offset_y :type int :default 0 :range [-50 50])
+  )
   (let* ((r (channel frame 0))
          (g (channel frame 1))
          (b (channel frame 2))