Take serial numbers from a pool the whole entry shares

Each station worked out its next number as the highest in its log plus one,
so two operators typing at the same moment both sent the same number and
one of the two contacts was wrong in the sponsor's log.

SerialNumberPool holds the highest number taken per band, which is N1MM's
QSONumbers array. A number is taken at the first character of the callsign,
as N1MM takes it, and ReserveNr tells the other stations before the contact
is worked. A number at or below one this station has used is answered with
the whole pool, N1MM's QSONRS, and the station that is behind counts on from
there. The pool also goes out when a contest is opened and when the numbers
are started again.

Contest.NumbersByBand says whether the numbers run per band or as one
sequence for the station. CQ WPX runs one per band for a multi-two and a
multi-multi; every other contest here runs one sequence. The band index in
the message is N1MM's, so the array lines up with theirs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015NZkFxHde1iifAQemFTHxf

docs/unfinished.md

@@ -365,10 +365,10 @@ which is the sending station's number, its computer name, the message type and
 the fields of that type. `%` and `~` cannot appear in a field, so N1MM writes
 `!` in their place. What is read and sent: contacts, edits, deletes, resyncs,
 `IAM`, the echo pair, chat, the pass frequency, transmit on and off, where a
-station is tuned, the time of a station's last contact, and the four messages a
-resync is made of. The rest of N1MM's forty-odd types — the score and sked windows, the
-log check, the spot lists, the serial-number pool — are passed over, which
-N1MM's own reader also does for a type it does not know.
+station is tuned, the time of a station's last contact, the two serial number
+messages, and the four messages a resync is made of. The rest of N1MM's
+forty-odd types — the score and sked windows, the log check, the spot lists —
+are passed over, which N1MM's own reader also does for a type it does not know.
 
 **The version has to match.** N1MM compares the version in the beacon with its
 own and, when they differ, puts up "Software versions must match. Update N1MM+."