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/posterize.sexp

@@ -1,8 +1,9 @@
 ;; Posterize effect - reduces color levels
-;; @param levels int [2, 32] default 8
 
 (define-effect posterize
-  ((levels 8))
+  :params (
+    (levels :type int :default 8 :range [2 32])
+  )
   (let ((step (floor (/ 256 levels))))
     (map-pixels frame
       (lambda (x y c)