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/scanlines.sexp

@@ -1,10 +1,11 @@
 ;; Scanlines effect - VHS-style horizontal line shifting
-;; @param amplitude float [0, 100] default 10
-;; @param frequency float [1, 100] default 10
-;; @param randomness float [0, 1] default 0.5
 
 (define-effect scanlines
-  ((amplitude 10) (frequency 10) (randomness 0.5))
+  :params (
+    (amplitude :type int :default 10 :range [0 100])
+    (frequency :type int :default 10 :range [1 100])
+    (randomness :type float :default 0.5 :range [0 1])
+  )
   (map-rows frame
     (lambda (y row)
       (let* ((sine-shift (* amplitude (sin (/ (* y 6.28) (max 1 frequency)))))