ImageVulkan kept its host-visible staging `buffer` (sized w*h, persistently
mapped) alive for the whole life of the image and Destroy() never freed it, so
static textures (e.g. Sponza albedo) pinned HOST_VISIBLE / small-BAR memory
forever.
Update now releases the staging via VulkanBuffer::DeferredClear() right after
recording the buffer→image copy — the same fence-keyed deletion queue
(#101/#102) the compressed Mesh path already uses — so it is freed once the
upload submit's frame has cleared. A new `streamed` flag (set by FontAtlas)
keeps the persistent map for images re-uploaded from a CPU-side staging buffer
every frame; its uploads go through UpdateRegion, which never releases. Destroy
now also frees any staging the image still owns, gated on a live handle so a
released static texture can't double-free.
Adds tests/ImageStagingRelease driving the real upload + readback on a headless
device: asserts the staging is enqueued (not pinned), the image reads back
byte-equal (released staging outlived the submit), the entry retires only after
framesInFlight frames, and a streamed image keeps then frees its staging.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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