Walk the QTC window the way N1MM walks it

Four changes to the QTC window, three of them N1MM's behaviour and one an
addition.

Tab, enter and space all move to the next box the operator types in, and shift
and tab move back. The walk covers those boxes and nothing else. N1MM handles
the three keys in every box itself — `QTCLinesA_KeyDown` and its two siblings,
with `PreviewKeyDown` claiming Tab — rather than leaving them to the tab order,
so the cursor never lands on a switched-off line or on a button beside one.
Tab was going to Avalonia's default walk, which covered every control in the
window.

The header now says how many lines take traffic. `QTCHeader_LostFocus` reads
the count after the last `/` and hands it to `ColorizeCWRXFields`, which enables
that many lines and switches off the rest, capped by how many of the station's
ten are still free. All ten were live whatever the header said. Nothing already
typed is cleared, which N1MM does under `DontErase`.

A time typed short is filled in from the line above as the cursor leaves the
box: `52` under `1148` reads as `1152`. `QTCLinesA_LostFocus` takes as many
characters off the front of the line above as the typed part is missing, and
drops a colon whatever the length. `QtcTraffic.FillTime` is that rule.

The addition: while the cursor is in a QTC callsign box the check window
answers for that call rather than the entry window's, so a call taken down in
traffic is checked the same way one being worked is. `CheckWindowSources`
takes the call from `Elsewhere` when the QTC window has set it.

Checked by driving the window: header `4/5` left five lines live and read
"1 of 5", tab walked header to time to call to serial to the next line's time,
shift and tab walked back, `52` on the second line became `1152`, and the check
window listed W1AW while `W1A` was typed in a QTC line.

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

README.md

@@ -731,6 +731,26 @@ being worked, and never more than the ten any station may have — and cannot be
 edited. On CW and SSB the direction is fixed by the rules; on RTTY traffic goes
 both ways and the window has a switch.
 
+Tab, enter and space all move to the next box the operator types in, and shift
+and tab move back. The walk covers those boxes and nothing else: N1MM handles
+the three keys itself in every box rather than leaving them to the tab order,
+so the cursor never lands on a switched-off line or on a button beside one.
+
+The header says how many lines take traffic. `4/5` is series 4 with five QTCs
+in it, so five lines are live and the other five are switched off; a station
+with fewer than five of its ten left gets that many instead. Nothing already
+typed is cleared when the header changes.
+
+A time typed short is filled in from the line above as the cursor leaves the
+box: `52` under `1148` reads as `1152`. As many characters are taken off the
+front of the line above as the typed part is missing, which is N1MM's rule, and
+a colon is dropped whatever the length.
+
+One thing here that N1MM does not do: while the cursor is in a QTC callsign
+box, the check window answers for that call rather than for the one in the
+entry window, so a call taken down in traffic is checked the same way one being
+worked is.
+
 Right-click the window for **Setup**: how many QTCs a series carries, whether
 the RX Ready and Cfm steps are stopped at, whether Hdr Agn and Agn clear what
 they ask for again, and the CW messages below. The defaults are N1MM's — both