Add loop accumulator node

README.md

@@ -15,6 +15,7 @@ The node is registered as:
 - `prompt_builder / SxCP For Loop Start`
 - `prompt_builder / SxCP For Loop End`
 - `prompt_builder / SxCP Loop Append`
+- `prompt_builder / SxCP Accumulator`
 - `prompt_builder / SxCP Category Preset`
 - `prompt_builder / SxCP Cast Control`
 - `prompt_builder / SxCP Generation Profile`
@@ -118,6 +119,22 @@ want to resume a loop without changing index-derived seeds or row numbers.
 you want to update each iteration. They are separate from the collector and grow
 dynamically in the UI as you connect them.
 
+`SxCP Accumulator` stores outputs across executions under a `store_key` or the
+node id. Put it after an image-producing step inside or after a loop, connect the
+generated `image`, and connect `For Loop Start.index` to `entry_id` when you want
+reruns to replace the same row instead of appending duplicates. Its outputs are:
+
+- `collection`: all stored values, or images when no explicit `value` is wired.
+- `image_batch`: all stored images as one ComfyUI image batch when they share
+  the same height and width. Set `image_batch_mode=resize_to_first` if you want
+  mixed sizes resized into one batch.
+- `image_list`: a ComfyUI list output containing each stored image separately.
+- `image_batch_1..4`: same-size grouped batches for mixed-format runs, so a
+  square group and a portrait group can be saved or processed separately.
+
+ComfyUI image batches require matching dimensions. For mixed image formats, use
+`image_list` or the grouped `image_batch_1..4` outputs instead of `image_batch`.
+
 ## Character Profiles
 
 `SxCP Woman Slot` and `SxCP Man Slot` are the scalable per-participant control