feat: Sidecar + Save Image (Sidecars) nodes + registration

Sidecar chains {content,name,ext} specs over a SIDECAR-typed link; the save node
mirrors SaveImageKJ (folder_paths.get_save_image_path) and writes each sidecar as
base+name+ext next to the image. build_plan validates the whole chain before any
I/O so duplicates/bad extensions write nothing.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_sidecar_node.py

@@ -0,0 +1,34 @@
+from gates import sidecar_node as sn
+
+
+def test_sidecar_run_builds_chain():
+    node = sn.Sidecar()
+    (chain,) = node.run(content="hello", name="", extension=".txt", sidecar=None)
+    assert chain == [{"content": "hello", "name": "", "ext": ".txt"}]
+    (chain2,) = node.run(content="{}", name="meta", extension=".json", sidecar=chain)
+    assert len(chain2) == 2
+    assert chain2[1] == {"content": "{}", "name": "meta", "ext": ".json"}
+
+
+def test_sidecar_io_shape():
+    assert sn.Sidecar.RETURN_TYPES == ("SIDECAR",)
+    it = sn.Sidecar.INPUT_TYPES()
+    assert "content" in it["required"]
+    assert "name" in it["required"]
+    assert "extension" in it["required"]
+    assert "sidecar" in it["optional"]
+
+
+def test_save_node_io_shape():
+    assert sn.SaveImageSidecars.OUTPUT_NODE is True
+    assert sn.SaveImageSidecars.RETURN_TYPES == ()
+    it = sn.SaveImageSidecars.INPUT_TYPES()
+    for k in ("images", "filename_prefix", "output_folder"):
+        assert k in it["required"]
+    assert "sidecar" in it["optional"]
+
+
+def test_mappings_present():
+    assert "Sidecar" in sn.NODE_CLASS_MAPPINGS
+    assert "SaveImageSidecars" in sn.NODE_CLASS_MAPPINGS
+    assert sn.NODE_DISPLAY_NAME_MAPPINGS["SaveImageSidecars"]