Lets a fresh repeater install pick the pymc_usb (USB-CDC) or pymc_tcp
(Wi-Fi/Ethernet) external modem from the first-run /setup wizard
instead of requiring the user to hand-edit config.yaml after install.
radio-settings.json gets two new hardware entries; setup_wizard()
in api_endpoints.py handles them in dedicated branches that mirror
the existing KISS pattern (placeholders if the SPA doesn't yet send
modem-specific inputs, request body overrides if it does).
For pymc_tcp the wizard writes a sentinel host placeholder
('REPLACE_WITH_MODEM_HOST') so the YAML stays valid; on startup
get_radio_for_board() then errors with a clear pointer at
pymc_tcp.host (existing behavior from the PR #240 branch). pymc_usb
defaults to /dev/ttyACM0 at 921600 baud — matches the USB-CDC
device path documented in pymc_usb's README + pymc_driver.
Five new tests in tests/test_setup_wizard_pymc.py verify both
default and overridden code paths plus a KISS regression guard.
Lloyd