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/saturation.sexp

@@ -1,7 +1,8 @@
 ;; Saturation effect - adjusts color saturation
-;; @param amount float [0, 3] default 1
 ;; Uses vectorized shift-hsv primitive for fast processing
 
 (define-effect saturation
-  ((amount 1))
+  :params (
+    (amount :type int :default 1 :range [0 3])
+  )
   (shift-hsv frame 0 amount 1))