feat(wifi): allow single WiFi channel monitoring

[wnagele]

What

Adds --channel CH to both dump3411.py and the standalone wifi_feeder.py: pin the monitor-mode radio to a single fixed 2.4 GHz channel (1–11) instead of hopping 1–11. --channel-dwell is ignored in this mode.

Why

The default channel hopper spreads reception across eleven channels (0.2 s dwell each), so a single stationary transmitter is only heard a fraction of the time. When the transmitter's channel is already known — a bench test, or a fixed site you have characterised — full dwell on that channel means no frames lost to hops. Trade-off: the other ten channels go dark, so keep hopping for discovery and pin only for targeted monitoring.

How

• WiFiFeeder accepts a channel argument; when set, no ChannelHopper thread is created and run() puts the interface on that channel once via the existing set_channel() (log line: Fixed channel: N (hopping disabled)). The shutdown path is guarded for the absent hopper.
• --channel is validated against CHANNELS_24 in both entry points (dump3411.py and wifi_feeder.py's standalone main()), with a clear error outside 1–11.
• README flag table documents the option; CHANGELOG entry under Added.