Textual's ContentSwitcher expects regular Widget/Container children,
not Screen subclasses. Screen's on_mount fires for all children at
once regardless of visibility, so demo workers for all 5 modes
started simultaneously and competed for the bridge.
Container children get proper on_show/on_hide from ContentSwitcher's
visibility toggling — only the active panel's worker runs.
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.
