chore: import upstream snapshot with attribution

docs/md/how_to/python/virtual_server/duckdb.md

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+# DuckDB Virtual Server
+
+Perspective provides a built-in virtual server for
+[DuckDB](https://duckdb.org/), allowing `<perspective-viewer>` clients to query
+a server-side DuckDB database over WebSocket.
+
+For browser-only usage via DuckDB-WASM, see the
+[JavaScript DuckDB guide](../../javascript/virtual_server/duckdb.md).
+
+## Installation
+
+```bash
+pip install perspective-python duckdb
+```
+
+## Usage
+
+Create a server that exposes a DuckDB database to browser clients:
+
+```python
+import duckdb
+import tornado.web
+import tornado.ioloop
+from perspective import DuckDBVirtualServer
+from perspective.handlers.tornado import PerspectiveTornadoHandler
+
+# Create DuckDB connection and load data
+conn = duckdb.connect()
+conn.execute("CREATE TABLE my_table AS SELECT * FROM 'data.parquet'")
+
+# Create virtual server backed by DuckDB
+server = DuckDBVirtualServer(conn)
+
+# Serve over WebSocket
+app = tornado.web.Application([
+    (r"/websocket", PerspectiveTornadoHandler, {"perspective_server": server}),
+])
+
+app.listen(8080)
+tornado.ioloop.IOLoop.current().start()
+```
+
+Connect from the browser:
+
+```javascript
+const websocket = await perspective.websocket("ws://localhost:8080/websocket");
+const table = await websocket.open_table("my_table");
+document.getElementById("viewer").load(table);
+```
+
+## Examples
+
+- [Python DuckDB example](https://github.com/perspective-dev/perspective/tree/master/examples/python-duckdb-virtual)