Spanning-tree on Linksys srw2008
After finally admitting defeat I had to boot into Windows for managing my Linksys SRW2008 switch through its WebView, the reason being I could not find out how to enable regular spanning-tree on the switch.
Having enabled it through its IE-specific web UI, I logged in to the (rather poorly documented) CLI afterwards to check what had changed in the config. And the single, magic configuration line that enabled spanning-tree on the switch was this one:
spanning-tree
*sigh*