Started to write another test case to blurt out OOK energenie messages. ...

This is only a test case.
But it has highlighted a whole load of discrepancies in the original C
code to do with payload length calculation, and the 'magic' zero byte
at the start of the buffer for stuffing the SPI register address in.

Got the radio chirping again after a long period of not working at all

hrfm69_test.c now sending a tone. ...

Discovered that 1/5 times it sticks waiting for TXREADY.
This might suggest an unresolved sequencing error in startup.

Built a simple C test harness to start testing the radio with