Single shadow depth image shared across MAX_FRAMES=2 in-flight GPU frames
caused a race: frame N's main pass reads shadow map while frame N+1's
shadow pass clears and writes it, producing visible flashing standing
still and while moving.
Fix: give each in-flight frame its own VkImage, VmaAllocation, VkImageView,
and VkFramebuffer for the shadow depth attachment. renderShadowPass() now
indexes all shadow resources by getCurrentFrame(), and layout transitions
track per-frame state in shadowDepthLayout_[frame]. Cleanup loops over
MAX_FRAMES=2. Descriptor sets already written per-frame; updated shadow
image view binding to use the matching per-frame view.
edd7e5e591