Add spare Vulkan RT scene slot for pipelining - #156

Summary

• add a spare Vulkan hardware RT scene slot beyond the two in-flight frame slots
• avoid reusing an RT scene slot while older in-flight frames may still reference it
• remove the default active-slot initialization so the active scene is only set once a real RT scene has been built

Why

The pipelined Vulkan RT path previously allocated only kMaxFramesInFlight scene slots. With two frames in flight, that left no spare slot to build the next TLAS while both current frames were still using previous scene snapshots. That can produce alternating-frame corruption and invalid lighting, which matched the observed white/incorrect every-other-frame behavior.

Validation

• cmake --build C:/Users/frede/github/Frame/build/windows --config Debug --target FrameVulkan FrameVulkanTest
• 02_Dragon.exe --device=vulkan --auto_exit_seconds=3

Notes

• this is a narrow follow-up to PR Pipeline Vulkan hardware RT scene updates #155
• it does not attempt to change the rest of the pipelining design, only the RT scene slot reuse hazard