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_fx.sexp

@@ -0,0 +1,51 @@
+;; ASCII Art FX - converts image to ASCII characters with per-character effects
+
+(define-effect ascii_art_fx
+  :params (
+    ;; Basic parameters
+    (char_size :type int :default 8 :range [4 32]
+      :desc "Size of each character cell in pixels")
+    (alphabet :type string :default "standard"
+      :desc "Character set to use")
+    (color_mode :type string :default "color"
+      :choices [color mono invert]
+      :desc "Color mode: color, mono, invert, or any color name/hex")
+    (background_color :type string :default "black"
+      :desc "Background color name or hex value")
+    (invert_colors :type int :default 0 :range [0 1]
+      :desc "Swap foreground and background colors (0/1)")
+    (contrast :type float :default 1.5 :range [1 3]
+      :desc "Character selection contrast")
+
+    ;; Per-character effects
+    (char_jitter :type float :default 0 :range [0 20]
+      :desc "Position jitter amount in pixels")
+    (char_scale :type float :default 1.0 :range [0.5 2.0]
+      :desc "Character scale factor")
+    (char_rotation :type float :default 0 :range [0 180]
+      :desc "Rotation amount in degrees")
+    (char_hue_shift :type float :default 0 :range [0 360]
+      :desc "Hue shift in degrees")
+
+    ;; Modulation sources
+    (jitter_source :type string :default "none"
+      :choices [none luminance inv_luminance saturation position_x position_y position_diag random center_dist]
+      :desc "What drives jitter modulation")
+    (scale_source :type string :default "none"
+      :choices [none luminance inv_luminance saturation position_x position_y position_diag random center_dist]
+      :desc "What drives scale modulation")
+    (rotation_source :type string :default "none"
+      :choices [none luminance inv_luminance saturation position_x position_y position_diag random center_dist]
+      :desc "What drives rotation modulation")
+    (hue_source :type string :default "none"
+      :choices [none luminance inv_luminance saturation position_x position_y position_diag random center_dist]
+      :desc "What drives hue shift modulation")
+  )
+  (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-fx frame chars colors luminances char_size
+                         color_mode background_color invert_colors
+                         char_jitter char_scale char_rotation char_hue_shift
+                         jitter_source scale_source rotation_source hue_source)))