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/layer.sexp

@@ -3,7 +3,12 @@
 ;; Params: x, y (position), opacity (0-1), mode (blend mode)
 
 (define-effect layer
-  ((x 0) (y 0) (opacity 1.0) (mode "alpha"))
+  :params (
+    (x :type int :default 0)
+    (y :type int :default 0)
+    (opacity :type float :default 1.0)
+    (mode :type string :default "alpha")
+  )
   (let [bg (copy frame-a)
         fg frame-b
         ;; Resize fg if needed to fit