Add README with usage docs, inputs reference, and metadata info

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

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+# ComfyUI WanVideo Save Merged Model
+
+A ComfyUI custom node that saves a [WanVideo Wrapper](https://github.com/kijai/ComfyUI-WanVideoWrapper) diffusion model — including merged LoRAs — as a standalone `.safetensors` checkpoint.
+
+This lets you bake LoRA weights into a base model once and reload the result directly, without re-merging every time.
+
+## Installation
+
+Clone or copy this repository into your ComfyUI `custom_nodes` directory:
+
+```bash
+cd ComfyUI/custom_nodes
+git clone <repo-url> Comfyui_WW_msave
+```
+
+Restart ComfyUI. The node will appear under the **WanVideoWrapper** category.
+
+## Node: WanVideo Save Merged Model
+
+### Inputs
+
+| Input | Type | Required | Description |
+|-------|------|----------|-------------|
+| `model` | `WANVIDEOMODEL` | Yes | Model output from the **WanVideo Model Loader** node (with LoRA merged) |
+| `filename_prefix` | `STRING` | Yes | Filename for the saved file (default: `merged_wanvideo`) |
+| `save_dtype` | Dropdown | No | Cast weights before saving: `same`, `bf16`, `fp16`, `fp32`. Defaults to `same` (keeps current dtype). Set explicitly if the model was loaded in fp8. |
+| `custom_path` | `STRING` | No | Absolute path to a save directory. Leave empty to save in `ComfyUI/models/diffusion_models/` |
+
+### Usage
+
+1. Set up the **WanVideo Model Loader** with a base model and one or more LoRAs (with `merge_loras` enabled)
+2. Connect the `model` output to **WanVideo Save Merged Model**
+3. Set a filename prefix and optionally choose a dtype and save path
+4. Queue the prompt — the merged checkpoint will be saved as `<filename_prefix>.safetensors`
+
+```
+[WanVideo Lora Select] --> lora
+                             |
+[WanVideo Model Loader] --> model --> [WanVideo Save Merged Model]
+```
+
+The saved file can then be loaded directly by the WanVideo Model Loader as a regular diffusion model, without needing the original LoRA files.
+
+### Metadata
+
+The saved `.safetensors` file includes metadata for traceability:
+
+- `source_model` — name of the base model
+- `merged_loras` — JSON list of merged LoRA names and strengths
+- `save_dtype` — dtype used when saving
+
+You can inspect this metadata with any safetensors viewer or with Python:
+
+```python
+from safetensors import safe_open
+with safe_open("merged_wanvideo.safetensors", framework="pt") as f:
+    print(f.metadata())
+```
+
+### Notes
+
+- Existing files are never overwritten — a counter suffix (`_1`, `_2`, ...) is appended automatically
+- The `custom_path` input only accepts absolute paths; relative paths are ignored and the default location is used
+- Saving a 14B model in bf16 produces a ~28 GB file — make sure you have enough disk space
+- If the model was quantized to fp8, use `save_dtype` to cast back to `bf16` or `fp16` for a more portable checkpoint
+
+## Requirements
+
+- [ComfyUI](https://github.com/comfyanonymous/ComfyUI)
+- [ComfyUI-WanVideoWrapper](https://github.com/kijai/ComfyUI-WanVideoWrapper)
+
+## License
+
+MIT