Keeping a 288 MB upload with each view's stages made them thrash against the stage
budget; not keeping it made every new window (a pan crossing a snap, the first
window of the Before side, any control that invalidates the sampled pixels) upload
and convert the source again, about 64 ms against 6-12 ms. A source too big for a
view's stages is now kept once for all views of the photo (shared_source), outside
the views' stages, and given back when the app idles (trim_pool(0)).
source_uploads and shared_source_bytes expose it for tests and diagnostics.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
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