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/ascii_zones.sexp

@@ -1,12 +1,13 @@
 ;; ASCII Zones effect - different character sets for different brightness zones
 ;; Dark areas use simple chars, mid uses standard, bright uses blocks
-;; @param char_size int [4, 32] default 8
-;; @param dark_threshold int [0, 128] default 80
-;; @param bright_threshold int [128, 255] default 180
-;; @param color_mode string default "color"
 
 (define-effect ascii_zones
-  ((char_size 8) (dark_threshold 80) (bright_threshold 180) (color_mode "color"))
+  :params (
+    (char_size :type int :default 8 :range [4 32])
+    (dark_threshold :type int :default 80 :range [0 128])
+    (bright_threshold :type int :default 180 :range [128 255])
+    (color_mode :type string :default "color")
+  )
   (let* ((sample (cell-sample frame char_size))
          (colors (nth sample 0))
          (luminances (nth sample 1))