Separate entry point (skywalker-tui) that reuses skywalker_lib.py
unchanged. Five RF modes: spectrum, scan, monitor, lband, track —
each with threaded USB bridge workers for non-blocking I/O.
Includes --demo mode with synthetic signal generation (Gaussian
peaks, noise floor, AGC simulation) for development without hardware.
Custom widgets: spectrum bar chart, rolling waterfall, signal gauge,
sparkline history, transponder table, device status bar.
