refactor: process isolation support for node replacement API

- Move REGISTERED_NODE_REPLACEMENTS global to NodeReplaceManager instance state
- Add NodeReplacement class to ComfyAPI_latest with async register() method
- Deprecate module-level register_node_replacement() function
- Call register_replacements() from comfy_entrypoint()

This enables pyisolate compatibility where extensions run in separate
processes and communicate via RPC. The async API allows registration
calls to cross process boundaries.

Refs: TDD-002
Amp-Thread-ID: https://ampcode.com/threads/T-019c2b33-ac55-76a9-9c6b-0246a8625f21

app/node_replace_manager.py

@@ -6,18 +6,39 @@ from typing import TYPE_CHECKING
 if TYPE_CHECKING:
     from comfy_api.latest._node_replace import NodeReplace
 
-REGISTERED_NODE_REPLACEMENTS: dict[str, list[NodeReplace]] = {}
-
-def register_node_replacement(node_replace: NodeReplace):
-    REGISTERED_NODE_REPLACEMENTS.setdefault(node_replace.old_node_id, []).append(node_replace)
-
-def registered_as_dict():
-    return {
-        k: [v.as_dict() for v in v_list] for k, v_list in REGISTERED_NODE_REPLACEMENTS.items()
-    }
 
 class NodeReplaceManager:
+    """
+    Manages node replacement registrations.
+    
+    Stores replacements as instance state (not module-level globals) to support
+    process isolation via pyisolate, where extensions run in separate processes
+    and communicate via RPC.
+    """
+    
+    def __init__(self):
+        self._replacements: dict[str, list[NodeReplace]] = {}
+    
+    def register(self, node_replace: NodeReplace):
+        """Register a node replacement mapping."""
+        self._replacements.setdefault(node_replace.old_node_id, []).append(node_replace)
+    
+    def get_replacement(self, old_node_id: str) -> list[NodeReplace] | None:
+        """Get replacements for an old node ID."""
+        return self._replacements.get(old_node_id)
+    
+    def has_replacement(self, old_node_id: str) -> bool:
+        """Check if a replacement exists for an old node ID."""
+        return old_node_id in self._replacements
+    
+    def as_dict(self):
+        """Serialize all replacements to dict."""
+        return {
+            k: [v.as_dict() for v in v_list] 
+            for k, v_list in self._replacements.items()
+        }
+    
     def add_routes(self, routes):
         @routes.get("/node_replacements")
         async def get_node_replacements(request):
-            return web.json_response(registered_as_dict())
+            return web.json_response(self.as_dict())