Add SaveSnapshot node with visual separation and rolling limit

Introduce a SaveSnapshot custom node that triggers snapshot captures
via WebSocket. Node-triggered snapshots are visually distinct in the
sidebar (purple left border + "Node" badge) and managed with their
own independent rolling limit (maxNodeSnapshots setting), separate
from auto/manual snapshot pruning. Node snapshots skip hash-dedup
so repeated queue runs always capture.

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

snapshot_node.py

@@ -0,0 +1,39 @@
+from server import PromptServer
+
+
+class _AnyType(str):
+    def __ne__(self, other):
+        return False
+
+
+ANY_TYPE = _AnyType("*")
+
+
+class SaveSnapshot:
+    CATEGORY = "Snapshot Manager"
+    FUNCTION = "execute"
+    RETURN_TYPES = (ANY_TYPE,)
+    OUTPUT_NODE = True
+
+    @classmethod
+    def INPUT_TYPES(cls):
+        return {
+            "required": {
+                "value": (ANY_TYPE, {}),
+                "label": ("STRING", {"default": "Node Trigger"}),
+            }
+        }
+
+    @classmethod
+    def VALIDATE_INPUTS(cls, input_types):
+        return True
+
+    @classmethod
+    def IS_CHANGED(cls, *args, **kwargs):
+        return float("NaN")
+
+    def execute(self, value, label):
+        PromptServer.instance.send_sync(
+            "snapshot-manager-capture", {"label": label}
+        )
+        return (value,)