Name the band beside every frequency, and map who was heard

Two additions, both about turning a number into something meaningful.

A band column.  Next to every frequency -- on the live display, in the
line-per-hit output, in saunterbrowse's list and details -- is the name of
the band it falls in.  421 MHz is the 70 cm amateur band, and being told
so is quicker than remembering where the edges are.

The names come from the existing preset table, so there is one band plan
to keep right rather than two, but naming is not the job that table was
shaped for: several presets cover any frequency, some of them whole-tuner
sweeps that say nothing.  So the candidates are ranked.  Sweeps and the
"-complete" duplicates are dropped outright.  The narrowest of what is
left wins, because it says the most -- 146.52 MHz comes back as the 2 m
simplex calling channel rather than as the whole 2 m band.  Two exceptions
where the narrowest would be the wrong answer: ISM yields to the
allocation it shares (433.92 is 70 cm first, 915 is 33 cm first), and
shortwave broadcast yields to amateur where the two overlap, because
3.9-4.0 and 7.2-7.3 MHz are Region 1 and 3 broadcast but Region 2 amateur,
and this plan is documented as Region 2.  6 MHz really is 49 m shortwave
and is left alone.

The name is written into each capture's sidecar, so it travels with the
recording and an edit to the plan later cannot rewrite history, and
saunterbrowse searches on it: /70 cm finds the band without anyone having
to remember 420-450 MHz.

A map.  A licence says where its holder is, so a list of callsigns is also
a map.  Callsigns heard during a scan are now looked up as the transcripts
come in, announced on the display, and written to callsigns.kml in the
output directory; saunterbrowse --kml builds the same file from recordings
already on disk, and the two continue one map rather than starting two.

One placemark per station, not one per transmission: the same repeater
heard twenty times in an evening is one operator, and twenty pins on one
rooftop would say less than one.  Each pin carries the callsign, the
licensee, the town, the grid square, and every frequency and time it was
heard on.  The file is read back on open and added to, so later scans
build it up rather than replacing it.

Where a licence has no coordinates the grid square's centre is used and
the placemark says so -- a square is kilometres across where an address is
a street.  A callsign with no licence at all is still recorded, in a
folder that starts switched off, because that a station was heard is worth
keeping even when nothing says where.  A file already there that is not
readable as KML is never overwritten.

Also fixed along the way:

- The hit list's "no signals recorded yet" placeholder was one cell short
  of its row, so it landed in the SNR column and wrapped, making the panel
  taller than the layout had budgeted for and scrolling the display off a
  short terminal.  The identification column can no longer wrap either,
  which is what _hit_capacity has always assumed.

- Licence lookups now record coordinates.  The cache is versioned so that
  entries written before this are asked about again, rather than pinning
  every station to its grid square for good.

- CallsignBook.wait dropped joined threads; an all-night scan calls it
  after every transcript and the list only ever grew.

- Tests redirect XDG_CACHE_HOME, so a run no longer reads or writes the
  real lookup cache.

676 -> 761 tests.

bandsaunter/tui.py

@@ -559,7 +559,23 @@ It is recognised by its symbol rate and its refusal to pause, named on screen,
 and skipped within a second or two.  Turn 'Skip trunk control channels' off
 only if you are collecting them for a decoder.  If digital voice calls are
 being skipped by mistake, raise 'Control channel patience'."""),
-    "9": ("Keys during a scan", """
+    "9": ("Callsigns and the map", """
+Anyone who identifies themselves in a transcript is picked out and looked up in
+the FCC's published licence register: the name, the town, the class of licence.
+The callsign is the only thing sent, and each one is asked about once and then
+remembered, so a net recorded night after night is looked up once.
+
+Because a licence says where its holder is, the same information is written as
+a map — callsigns.kml in the output directory — with one pin per station,
+carrying the licensee, the town, and every frequency and time you heard them.
+It opens in Google Earth, QGIS, Marble and OsmAnd, and later scans add to it
+rather than starting it over, so it fills in as a picture of what your aerial
+can reach.
+
+Turn 'Look callsigns up' off to keep the scan entirely offline; callsigns are
+still found, and named by country from their prefix.  Clear 'Map file' to stop
+writing the map.  US licence records are public, and include addresses."""),
+    "10": ("Keys during a scan", """
 q   stop the scan
 p   pause and resume
 s   skip the signal being recorded and carry on sweeping