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_art.sexp

@@ -1,14 +1,16 @@
 ;; ASCII Art effect - converts image to ASCII characters
-;; @param char_size int [4, 32] default 8
-;; @param alphabet string default "standard"
-;; @param color_mode string default "color"
-;; @param contrast float [1, 3] default 1.5
-;; @param background list default (0 0 0)
 
 (define-effect ascii_art
-  ((char_size 8) (alphabet "standard") (color_mode "color") (contrast 1.5) (background (list 0 0 0)))
+  :params (
+    (char_size :type int :default 8 :range [4 32])
+    (alphabet :type string :default "standard")
+    (color_mode :type string :default "color" :desc ""color", "mono", "invert", or any color name/hex")
+    (background_color :type string :default "black" :desc "background color name/hex")
+    (invert_colors :type int :default 0 :desc "swap foreground and background colors")
+    (contrast :type float :default 1.5 :range [1 3])
+  )
   (let* ((sample (cell-sample frame char_size))
          (colors (nth sample 0))
          (luminances (nth sample 1))
          (chars (luminance-to-chars luminances alphabet contrast)))
-    (render-char-grid frame chars colors char_size color_mode background)))
+    (render-char-grid frame chars colors char_size color_mode background_color invert_colors)))