[Aschenbath]

When a timed window feeds an async def sink, the timer callback and subsequent inputs can await the same coroutine more than once, raising RuntimeError: cannot reuse already awaited coroutine. Convert each window's downstream results to a shared Future once, so concurrent and later inputs wait for the same work. Apply the same fix to timed_window_unique.

Fixes #284.

Nine regression/control cases cover both window types, both unique retention modes, native coroutines, Tasks, Tornado Futures, concurrent producers, completed work, two successive windows and a synchronous sink branch. On the original source, the three native-coroutine cases fail; all nine pass with the change on Windows CPython 3.10.22 and 3.12.15. Independent agent review passed 240 assertions, including exception propagation and synchronous callers using a background event loop. Flake8, wheel/sdist builds and installed-wheel checks pass.

The full suite is not green on this Windows host. The complete core module reports 130 passed / 4 failed with the patch; unchanged source reports 120 passed / 5 failed. All four patched failure cases also fail on the unchanged source across the baseline runs, involving existing timing/order assertions. Full-project runs on both versions reach 99% before the same test_process_str stalls and the 30-second watchdog stops execution. Other platforms and the upstream CI matrix were not run locally.

AI disclosure: prepared and tested with OpenAI Codex under the account owner's authorization, with independent review by a separate coding agent. No human review is claimed.