Stop reporting playback on a clip that ended during the play call

[danielchalmers]

PlayAsync sets IsPlaying once its play operation completes, with nothing checking whether the clip is still running by then. A clip that reaches its end while that operation is in flight raises Ended, whose handler correctly clears IsPlaying, and then the play call sets it straight back. The transport is left claiming to play a clip parked on its last frame, and nothing clears it until you press something else.

Found while instrumenting an end-of-clip measurement during the Flyleaf 9.0 upgrade (#74). It stalled once in eight runs, identically on FlyleafLib 3.10.4 and 3.11.3, so this is pre-existing and unrelated to that upgrade, which is why it was held back into its own PR. The IsPlaying trace makes the ordering plain:

--- iteration 0 start (playing=True) ---
2ms     seeked to 00:00:13.8
1318ms  IsPlaying=False pos=00:00:13.8   (Ended fired, handler cleared it)
8019ms  IsPlaying=True  pos=00:00:15     (play call returned and set it back)

The window is not exotic. Play pressed near the end of a clip hits it, and so does a play queued behind an open that is still joining secondary cameras, which is what took eight seconds in the trace above.

The fix

A counter bumped by the Ended handler lets the play call tell that the clip finished underneath it and leave the settled state alone. FakeCameraPlayer gains a PlayCallback hook, mirroring the SeekCallback it already had for interleaving an action mid-call.

Two tests: one ends the clip from inside the play call and asserts the controller does not report playback, one asserts an ordinary resume still does. I verified the first fails without the guard, with Shouldly.ShouldAssertException : controller.IsPlaying.

374 tests green, dotnet format --verify-no-changes clean.