Put the APRS channel on the front page, and find it for you

The region was in the menu, in the receiver group, between the sample rate and
the invented channel.  That is the wrong place for it.  It is the setting that
decides whether anything is heard at all, and being on the wrong one sounds
exactly like having no aerial, so it does not belong a level down among the
things that make a working receiver work slightly better.

It is now on the front page of the APRS menu, named as well as numbered, and
so is the Listen line: "north-america 144.39 MHz" rather than "144.39 MHz",
because the number alone does not say whether it is the right one and the name
alone does not say what will be tuned.  A channel that is no region's says so
rather than claiming one.

The region and the frequency are separate settings -- somebody may want a
local packet network on neither -- which means they can be made to disagree.
Every place that chooses a region now goes through one function, so they
cannot.

And there is a search.  `bandsaunter aprs --find-channel`, or f in the menu,
listens on each region's channel in turn and prints what was on each, then
offers to use the busiest.  This answers the one question about APRS that
cannot be answered on any single frequency, because the answer *is* a
frequency: somebody who has just plugged a dongle in cannot tell a wrong
channel from a dead aerial, and that is worth a minute of listening rather
than an evening of doubt.

What it does not do is claim more than it found.  A quiet channel is not proof
of an empty one -- a fixed station beacons every half hour -- so what it finds
is traffic, and when every channel comes back silent it says that this is not
the same as an empty band and points at the aerial instead.

The invented channel now honours tuning, and only carries its stations on its
own frequency.  Without that the search would pass on a simulated band having
never searched anything, which is the kind of test that is worse than none.

Sixteen new tests against six deliberately broken builds.  The three that
searched a whole simulated band were doing the same expensive thing three
times over and now build their results directly, leaving one real end-to-end
search; that file went from nine and a half minutes to two.  Full suite 2612
passed.  Built as 2026-09-20_03.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016PsWPTweCT6pwxKngvVxcg

README.md

@@ -2299,9 +2299,48 @@ EVENT1      fire             47.5900N 122.3300W                            5 km
 
 The frequency is agreed between amateurs rather than allocated, so it differs
 by region and **there is no way to discover it from the air**: on the wrong one
-you hear silence, not a bad signal. `--region` covers the common ones —
-`north-america` (144.390), `europe` (144.800), `australia` (145.175), `japan`,
-`brazil`, `thailand` — and `--frequency` takes a number for anything else.
+you hear silence, not a bad signal. That is the one fault that looks exactly
+like a dead aerial and is not, so it is the first thing to settle.
+
+```bash
+bandsaunter aprs --find-channel        # listen on each in turn, 20 s each
+bandsaunter aprs --region europe       # or set it, if you know
+```
+
+`--find-channel` answers the one question about APRS that cannot be answered
+on any single frequency, because the answer *is* a frequency:
+
+```
+what was on each channel
+region           frequency  packets  stations  strongest  used in
+north-america  144.390 MHz       21         6      38 dB  United States, Canada, Mexico
+europe         144.800 MHz        —         —          —  IARU Region 1, including the UK
+australia      145.175 MHz        —         —          —  Australia and New Zealand
+japan          144.640 MHz        —         —          —  Japan
+brazil         145.570 MHz        —         —          —  Brazil
+thailand       145.525 MHz        —         —          —  Thailand
+north-america had the most on it — 21 packets from 6 stations
+```
+
+A quiet channel is not proof of an empty one — a fixed station beacons every
+half hour — so what it finds is traffic, and what it misses is only the absence
+of traffic while it listened. It says so when everything comes back silent.
+
+In the menus the channel is on the front page rather than a level down with the
+gain and the sample rate, because it is the setting that decides whether
+anything is heard at all:
+
+```
+  l  Listen                 north-america 144.39 MHz, until stopped, everything, metric
+  c  Channel / region       north-america 144.39 MHz
+  f  Find the channel       listen on each region's in turn and see which has traffic
+  r  Read a log back        3 logs in ~/bandsaunter
+```
+
+**c** lists the regions with their frequencies and also takes a number in MHz
+for a channel that is not any region's — a local packet network, or one of the
+9600 baud links some areas run alongside the main one. **f** runs the search
+and offers to adopt whichever channel had the most on it.
 
 ### What it reads
 
@@ -2421,6 +2460,7 @@ version that can.
 | Gain | `--gain` | auto | tuner gain in dB, or automatic |
 | Sample rate | `--rate` | 240 kS/s | 96 kS/s is the least that holds the channel |
 | Region | `--region` | north-america | which APRS channel to listen on |
+| — | `--find-channel [SECONDS]` | — | listen on each region's in turn and say which has traffic |
 | Listen on | `--frequency`, `--freq` | 144.390 MHz | the exact frequency, if the region's is not what you want |
 | Invent a channel | `--simulate` / `--no-simulate` | no | stations that are not there |
 | Listen for | `--seconds` | until stopped | how long before stopping |
@@ -2447,8 +2487,9 @@ demodulator and the real parsers. Nothing touches the receiver.
 
 ### If nothing is heard
 
-**Check the region first.** It is the one fault that looks like a dead aerial
-and is not: on the wrong channel there is silence rather than a bad signal.
+**Check the region first**, and `bandsaunter aprs --find-channel` does it for
+you. It is the one fault that looks like a dead aerial and is not: on the wrong
+channel there is silence rather than a bad signal.
 
 Then give it time. A fixed station beacons every twenty or thirty minutes and a
 mobile every minute or two, so five minutes of an ordinary suburb might be