Creates the worker-side message handler. Holds the current map snapshot in
closure state; setMap replaces it, build runs buildChunkArrays plus
flat-normal computation and returns the arrays with a transfer list so the
result buffers move (not copy) back to the main thread.
Creates the worker-side message handler. Holds the current map snapshot in closure state;
setMapreplaces it,buildrunsbuildChunkArraysplus flat-normal computation and returns the arrays with a transfer list so the result buffers move (not copy) back to the main thread.