Blinking LEDs are a fundamental part of any board bringup
The first two attempts failed with the processor apparently halted. It turns out that the pullups and pulldowns I was using were way too large (27K) to deal with the load they were driving; it's unclear whether it was something weird about the OE pins they were pulling or the uninitialized GPIOs they were interfacing with on the RP2040. I just added some code on the RP2040 to pull the pins in the right direction at startup, and they get set up quickly enough for the machine to start.
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