The GpuSwapGate.release() was async and awaited in the dispatch `finally`
blocks. When a request was cancelled/interrupted mid-flight (client disconnect,
an interrupted text generation), `await self._swap_release(...)` in the finally
could itself be cancelled BEFORE it decremented the running counter — stranding
a gate slot. With `running` stuck > 0, `_pump()` never ran, so a video request
queued behind the interrupted text request was never granted: the GPU never
swapped even though the text engine had gone idle (observed: video stuck ~37min
while the text engine was idle for 11).

Fix: make release()/_pump() SYNCHRONOUS and drop the asyncio.Lock. Every critical
section is straight-line (no await between read and write), so under asyncio's
single thread they're already atomic — and a synchronous release from a `finally`
always completes even while the coroutine is being cancelled. acquire() keeps its
one `await` (t...