Retain the existing fingerprint's storage instead of allocating a fresh
owned slice on every update. The previous approach built a replacement
fingerprint before it knew whether the viewport changed, discarding it
for unchanged viewports.
The fingerprint now rebuilds in place after reserving viewport-sized
capacity and reports whether its contents changed. Unchanged viewport
updates skip the allocation, while changed viewports will usually reuse
the existing buffer for the lifetime of the active search.
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