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>

constructs/slice-every-n.sexp

@@ -12,9 +12,12 @@
 ;; Groups every N analysis times into one segment, calling reducer once per group
 
 (define-construct slice-every-n
-  "Group every N analysis beats into segments"
-  (analysis n)
-  ;; 'init' and 'reducer' come from keyword args
+  :params (
+    (analysis :type any :desc "Analysis data with :times")
+    (n :type int :default 4 :desc "Number of beats per segment")
+    (init :type any :default 0 :desc "Initial accumulator value")
+    (reducer :type any :desc "Reducer function (fn [acc i start end] ...)")
+  )
   ;; Reducer receives: (acc, i, start, end) where start/end are audio beat times
   ;; Reducer returns: {:source src :effects fx :acc new-acc}
   ;;   Optionally include :start/:end to override (e.g., for wrapping/randomizing)