Normalize input/output names across nodes

- SourcePrep output: source_clip → trimmed_clip (distinguish from input)
- MergeBack input: original_clip → source_clip (matches SourcePrep's
  input — same video wire from Load Video)
- MergeBack input: original_clip_2 → source_clip_2 (matches SourcePrep)
- Update README wiring diagram and tables

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

nodes.py

@@ -300,7 +300,7 @@ class VACESourcePrep:
     FUNCTION = "prepare"
     RETURN_TYPES = ("IMAGE", VACE_MODES, "INT", "INT", "MASK", "STRING", "VACE_PIPE")
     RETURN_NAMES = (
-        "source_clip", "mode", "split_index", "edge_frames",
+        "trimmed_clip", "mode", "split_index", "edge_frames",
         "inpaint_mask", "keyframe_positions", "vace_pipe",
     )
     OUTPUT_TOOLTIPS = (