feat(oakrender): render-process isolation S1 - dispatcher, scheduler, real worker

Per the M15 design (docs/zh/plans/riir/M15-render-process-isolation.md):

- ipc.rs moved into oakrender with protocol v2: hello_caps,
  render_batch, batch_accepted, frame_failed; main-process-assigned
  slots; BGRA8 slot format. POSIX shm verified to 1GiB on macOS.
- ProcessDispatcher: spawns oak-worker processes, handshake, stdio
  NDJSON control, shm segment lifecycle with generation-tagged keys,
  crash detection with bounded restart and frame redispatch, zero-copy
  ShmFrameRef delivery and copy counters.
- PreviewScheduler: interleaved batch claiming (frame % W per worker,
  no work stealing), seek > playback-distance > background priority,
  credit-based flow control, crash recovery.
- oak-worker renders for real: graph snapshot deserialization, montage
  decode+composite straight into the assigned shm slot, F32->BGRA8
  final conversion in-worker, OFX plugin executor installed in-worker,
  crash hooks for isolation testing.

Thread pool coexists for now (S2 removes it). Integration tests cover
two-worker zero-copy rendering, crash isolation with redelivery, and
real H.264 footage decode into slots.

crates/oaktask/src/render.rs

@@ -660,7 +660,7 @@ impl RenderTask {
 					oakrender::eval::render_produced_frame(time, params)
 						.map(TicketPayload::Video)
 				});
-				let arena = Arc::new(TicketArena::new(pool.clone(), producer));
+				let arena = Arc::new(TicketArena::new(Arc::new(pool.clone()), producer));
 				(arena, Some(pool))
 			}
 		};