Stop mounting children from blocking the parent's message queue

[trissim]

Mounting optional children queued a wait for composition on the parent's next-callback queue, which stopped wheel messages from bubbling through the parent while it waited. await_complete.py now has a shared AwaitCompletion base with a call_when_ready method, and AwaitMount and AwaitRemove use it, so the parent gets its callback only after the mount has finished. Waiters can still await the mount, and cancelling one waiter does not cancel startup. app.py and widget.py are updated to match, and tests are added to test_mount.py.

[trissim]

Integration review: the matched installed Toad App confirms the previous 219ms parent mount-completion callback hold is removed; history callbacks were about 1ms or less. Overall frame pacing still fails, so this is a targeted queue repair. Reviewed shared completion admission, distinct constructors, mount None result, cancellation and removal self-custody. The separate application viewport routing correction preserves native movement and nested controls. Proceeding with matched frontend integration.

[Copilot]

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