A flat colour is the one placeholder a client can paint with no decoding at all,
so it can cover the frames before a thumbhash canvas even renders. Extract it
from the same 100px thumbnail the two hash encoders already share, and expose it
as dominantColor on the native API.
Dominant means presence, not salience: the largest cluster wins, so a white
sleeve reports white. That is what a placeholder needs. An accent colour is the
opposite question and is deliberately not answered here - it depends on the
client's theme, and by the time a client needs one it has the cover pixels and
can compute whatever suits it.
Bins at 4 bits per channel, then merges perceptually-near bins in Oklab before
picking the winner: a gradient splits across adjacent bins and would otherwise
lose to a smaller flat region. Measured at 246us per image on the weakest target
hardware we care about (Celeron N5105), against ~15.8 images/s for the pipeline
around it.
The column goes into the feature's original migration rather than a new one:
computing it later would mean re-decoding every image in the library.
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