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The zoomed images look grainy as one would expect from raw data, but I would have expected them to do dark field subtraction for the chips to minimize this effect. Does anyone know if that's done (or expressly avoided) in this context, or why it might not be as helpful (e.g., for longer exposures)?

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Image creator here. We do dark field subtraction, as well as many other instrumental calibrations. What you are seeing is the fundamental photon noise. Because it is statistical in nature, you can never completely eliminate it. We could have chosen to put the black point in the image at a much higher flux level, but if you go to a high enough signal to noise level that you see no grain anywhere, you would miss out on so many interesting things that are still quite obvious to make out but are only 2-3 sigma above the noise.

Seems this will be done on the 'nightly' release cadence. Found on page 11 in this doc that I found from the wikipedia page:

https://docushare.lsstcorp.org/docushare/dsweb/Get/LSE-163/L...

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