Add browser_fetch helpers and async HTTP demos for Pyodide

Ship pyfetch-based fetch utilities in lib/, run asyncio scripts via
nest-asyncio in the worker, and add sample demos for HTTPS in the browser.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/static/bundled-demos/demo/aiohttp_fetch_demo.py

@@ -0,0 +1,32 @@
+"""Async HTTPS GET of Pyodide’s lock file (browser-safe).
+
+Older Pyodide + ``aiohttp`` examples used ``ClientSession`` here, but the
+Pyodide ``aiohttp`` wheel often raises ``RuntimeError('SSL is not supported.')``
+for ``https://`` — there is no real Python TLS stack in the worker; traffic must
+go through the browser’s ``fetch``.
+
+This demo uses the shared ``browser_fetch`` helpers (``pyodide.http.pyfetch``
+under the hood), same idea as ``async_fetch_demo.py``.
+
+CORS still applies — jsDelivr allows cross-origin GETs. ``print(..., flush=True)``
+helps batched worker stdout appear before the run finishes.
+"""
+
+import asyncio
+
+from browser_fetch import fetch_json
+
+
+async def main():
+    url = "https://cdn.jsdelivr.net/pyodide/v0.26.4/full/pyodide-lock.json"
+    print("Fetching", url, "via browser_fetch.fetch_json ...", flush=True)
+    data = await fetch_json(url)
+    info = data.get("info") or {}
+    packages = data.get("packages") or {}
+    print("pyodide-lock version:", info.get("version"), flush=True)
+    print("python runtime     :", info.get("python"), flush=True)
+    print("indexed packages   :", len(packages), flush=True)
+    print("aiohttp wheel in lock:", "aiohttp" in packages, flush=True)
+
+
+asyncio.run(main())

src/static/bundled-demos/demo/async_fetch_demo.py

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+"""Async HTTP in the browser (Pyodide).
+
+``aiohttp`` and similar clients use OS sockets, which Wasm does not provide, so
+a ``session.get(...)`` can hang after ``print("Running")`` with no body.
+
+Use ``pyodide.http.pyfetch`` or the shared ``browser_fetch`` helpers. From an
+HTTPS editor page, use ``https://`` URLs (mixed content blocks ``http://``).
+
+Many sites do not send CORS headers, so the browser blocks the response even
+when the URL is valid. This demo uses jsDelivr JSON that allows cross-origin
+GET (same host Pyodide loads from).
+"""
+
+import asyncio
+
+from browser_fetch import fetch_json
+
+
+async def main():
+    print("Running")
+    url = "https://cdn.jsdelivr.net/pyodide/v0.26.4/full/repodata.json"
+    data = await fetch_json(url)
+    info = data.get("info") or {}
+    print("Fetched repodata; pyodide lock says:", info.get("version"), info.get("python"))
+
+
+asyncio.run(main())