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/emboss.sexp

@@ -1,9 +1,10 @@
 ;; Emboss effect - creates raised/3D appearance
-;; @param strength float [0.5, 3] default 1
-;; @param blend float [0, 1] default 0.3
 
 (define-effect emboss
-  ((strength 1) (blend 0.3))
+  :params (
+    (strength :type int :default 1 :range [0.5 3])
+    (blend :type float :default 0.3 :range [0 1])
+  )
   (let* ((kernel (list (list (- strength) (- strength) 0)
                        (list (- strength) 1 strength)
                        (list 0 strength strength)))