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/beat-alternate.sexp

@@ -6,8 +6,10 @@
 ;;   (def segments (beat-alternate beats-data (list video-a video-b)))
 
 (define-construct beat-alternate
-  "Alternate between sources on each beat"
-  (analysis sources)
+  :params (
+    (analysis :type any :desc "Analysis data with :times")
+    (sources :type any :desc "List of source nodes to alternate between")
+  )
   ;; Body: map over time pairs, return segment descriptors
   (let [times (get analysis :times)
         pairs (zip-pairs (cons 0 times))