Replace the old MockPins implementation, which borrowed the machine's symbol
table at call time and required users to set up __GPIO_MOCK_DATA and
__GPIO_MOCK_LAST variables, with a self-contained MockPins that maintains its
own reads queue and operation trace.

Add a GPIO_MOCK_INJECT command and a GPIO_MOCK_TRACE function
(registered only with --gpio-pins=mock) to pre-seed reads and dump the
trace, and add a --gpio-pins CLI flag to select between rppal, noop, and
mock backends.  Rewrite the gpio.bas integration test to use these
commands instead of the old magic variables.  Update all unit tests to
use the new make_mock_machine helper directly.