CommandBufferManager logic is buggy

[JeanPhilippeKernel]

The CommandBufferManager has several design and implementation issues that cause or crashes in a of the Vulkan renderer.

Race Conditions in Instant Commands
GetInstantCommandBuffer uses a condition variable to wait for m_instant_semaphore->GetState() == Idle, but lacks proper multi-producer protection. Multiple threads calling it simultaneously could:

• All pass the predicate check before any submission completes.
• Overwrite m_executing_instant_command flag inconsistently.
• Submit overlapping buffers to the same semaphore/fence, corrupting GPU synchronization.