uh oh
i think i figured out where the spam is coming from:
wss://relay.mostr.pub
the Fediverse/Mastodon bridge.
it appears that this bridge functions as a general purpose relay - not only a bridge - and the spam bots have discovered it.
i can't have it enabled on Damus' global timeline if it's going to continue doing that. the bridge should probably not be accepting anything that isn't at least interacting with Fediverse users.