docs(story-1.8): RecordedTransport docstring notes BaseException rationale

Final review concern: the docstring named the route/default type
(`Response | BaseException`) without explaining why BaseException
(not Exception) is the chosen union. Adds a one-paragraph note: the
choice lets test code express `asyncio.CancelledError`, `SystemExit`,
etc.; surfaces that these bypass `except Exception:`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: lesnik512

src/httpware/transports/recorded.py

@@ -25,6 +25,11 @@ class RecordedTransport:
     swap the route between calls via `add_route(...)` or construct a new
     transport per call.
 
+    Route and default values may be `BaseException` (not just `Exception`) so
+    test code can express `asyncio.CancelledError`, `SystemExit`, or
+    `KeyboardInterrupt` — useful for cancellation/shutdown propagation tests.
+    These do NOT get caught by user code's `except Exception:`.
+
     `stream()` raises NotImplementedError; streaming lands in Epic 4 (Story 4-1).
     """