Fail fast with a clear error on event loop reuse instead of hanging

A Stealthii instance binds its lock, browser, and contexts to whichever
asyncio event loop is running the first time it is used. Using it again
from a different loop previously hung forever (the lock/semaphore/
Playwright transport are all bound to the dead first loop and never
wake the waiting coroutine). Detected in rsearch's test suite, where
pytest-asyncio's default per-test-function event loop broke the
module-level Stealthii singleton shared across tests.

Now raises StealthUnavailableError immediately with an actionable
message. README documents the constraint and the pytest-asyncio config
fix (asyncio_default_test_loop_scope = session).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0116i7dYLNZTXNR7Lqf439bV

tests/test_client.py

@@ -1,7 +1,7 @@
 # retoor <retoor@molodetz.nl>
 import pytest
 
-from stealthii import Stealthii, StealthResponse
+from stealthii import Stealthii, StealthResponse, StealthUnavailableError
 from stealthii.client import _ContextSlot
 
 
@@ -69,6 +69,15 @@ async def test_stealth_response_shapes_like_aiohttp():
         assert r is resp
 
 
+@pytest.mark.asyncio
+async def test_get_lock_raises_clearly_on_event_loop_mismatch():
+    s = Stealthii()
+    s._get_lock()
+    s._loop = object()
+    with pytest.raises(StealthUnavailableError, match='different asyncio event loop'):
+        s._get_lock()
+
+
 def test_context_slot_starts_fresh():
     slot = _ContextSlot(context=object())
     assert slot.count == 0