SAMD21 SERCOM / UART problem: cannot enable RX and TX
Digging deeper, the issue was that CTRLA.MODE was set to external clock source (i.e. XCK on PAD[2] where no clock is connected), rather than an internal clock source.

Additionally, CTRLA.DORD was set to MSB-first, whereas my linux endpoint prefers LSB-first.

The gist has been updated for a working "Hello, World!"