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/fisheye.sexp

@@ -1,11 +1,12 @@
 ;; Fisheye effect - barrel/pincushion lens distortion
-;; @param strength float [-1, 1] default 0.3
-;; @param center_x float [0, 1] default 0.5
-;; @param center_y float [0, 1] default 0.5
-;; @param zoom_correct bool default true
 
 (define-effect fisheye
-  ((strength 0.3) (center_x 0.5) (center_y 0.5) (zoom_correct true))
+  :params (
+    (strength :type float :default 0.3 :range [-1 1])
+    (center_x :type float :default 0.5 :range [0 1])
+    (center_y :type float :default 0.5 :range [0 1])
+    (zoom_correct :type bool :default true)
+  )
   (let* ((w (width frame))
          (h (height frame))
          (cx (* w center_x))