Cleanup pass over the three preceding commits.
Tabulate the cosine terms in the UI blurhash decoder instead of calling Math.cos
per pixel per component: 248us -> 75us for a 32x32 decode, and an album grid
mounts one decoder per tile. Output is unchanged, which the pinned pixel specs
enforce. The Go encoder already tabulated the same terms.
Drop the dead paths that deriving components inside Encode left behind: the
zero-size guard in components, the post-downscale empty check, and the
no-AC-factor branch, which cannot be reached now that the counts are always at
least 1x9. The empty-image check moves ahead of the derivation, where it belongs.
In the queue mock, look up item_artwork by its existing iaKey rather than
scanning the map, and hold the lock across EnqueueIfMissing through a shared
unlocked helper instead of releasing it mid-operation. Extract the duplicated
drain-and-resolve block in the scanner specs into one helper.
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