feat: initial ComfyUI_UltimateSGUpscale

Recreates UltimateSDUpscale features using built-in ComfyUI nodes
(SplitImageToTileList, ImageMergeTileList) plus a small GenerateSeamMask
helper node.

- GenerateSeamMask: creates white-band mask at tile seam positions
- Workflow JSON: 22-node tiled upscale pipeline with:
  - Pass 1: model upscale + tiled img2img redraw
  - Pass 2: targeted seam fix using SetLatentNoiseMask
- Unit tests for seam mask generation

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

__init__.py

@@ -0,0 +1,11 @@
+from .seam_mask_node import GenerateSeamMask
+
+NODE_CLASS_MAPPINGS = {
+    "GenerateSeamMask": GenerateSeamMask,
+}
+
+NODE_DISPLAY_NAME_MAPPINGS = {
+    "GenerateSeamMask": "Generate Seam Mask",
+}
+
+__all__ = ["NODE_CLASS_MAPPINGS", "NODE_DISPLAY_NAME_MAPPINGS"]