read_packet needs to check for leftover data in self._buffer

I've noticed a strange buffering phenomena documented in https://forums.adafruit.com/viewtopic.php?f=60&t=144694

I've had a _quick glance_ at the code but my first guess is this is because `read_packet()` returns `None` if there's no data coming back from a presumably non-blocking `self.uart_rx()`. I think the lack of checking for data leftover in self._buffer from a previous call to `read_packet()` is the problem. That previous call could have picked up two (or more) commands and `read_packet()` _only returns first_ by design. I've not executed/single-stepped the code here, i'm just guessing from inspection but it seems to match observed behaviour.

Start by tracing read_packet(), especially how it consumes self._buffer and calls self.uart_rx(). Reproduce the case where one call receives multiple commands, then verify that subsequent calls process leftover buffered data instead of returning None when no new UART data arrives; the issue does not name a test file.

Written by the indexing model from the issue text.