Keep an excluded contact out of the score the way N1MM does

Three things were wrong with a contact the operator takes out of the claimed
score. The decompiled reference gives the rule in four places:

- `Contest.cs:437` returns zero points when `IsClaimedQso` is 0.
- `ContestInstance.cs:307` builds the where clause `CLAIMEDQSO = 1`, which the
  dupe search (`DBSupport.cs:1189`) and the multiplier search
  (`DBSupport.cs:1327`) both use, so an unclaimed contact takes nothing away
  from a later one.
- The score query (`DBSupport.cs:1807`) adds `ContactType <> 'D'`, which the
  other two do not. A contact marked as a dupe is therefore still searched for
  dupes and multipliers, and is left out of the score alone.
- `Contact.cs:1563`, `3307` and `3881` write `X-QSO:` rather than dropping the
  contact from the Cabrillo file.

`ContestLog` scored an unclaimed contact in full, and the Cabrillo writer left
it out of the file. `Qso.IsInScore` now holds the score rule and
`StatisticsPivot` uses it instead of its own copy.

The same reference also fixes the sent serial in three Cabrillo exporters.
ARRL 10M, Ukrainian DX and YO DX HF formatted the sent exchange from the
station rather than the contact, so a DX entrant exported `001` on every line.
N1MM reads the contact's own number: `CArrl10m.cs:122` writes it unpadded,
`CUkrainian.cs:232` and `CYOHFDX.cs:167` pad it to three digits.

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

README.md

@@ -495,6 +495,13 @@ save first. Points and the multiplier flags are shown but cannot be typed in —
 they are worked out from the rules every time the log changes, so anything typed
 there would be overwritten on the next edit.
 
+The dialog also has the checkbox that takes a contact out of the claimed score.
+An unclaimed contact scores no points and holds no multiplier, and the search
+for dupes and multipliers passes over it, so a later contact with the same call
+or the same multiplier counts in full. It still goes into the Cabrillo file,
+under `X-QSO:` rather than `QSO:` — that is what N1MM writes, and the sponsors
+read it as a contact the entrant is not claiming.
+
 Correcting the country prefix by hand changes the score. The country file is a
 best guess for calls it has no rule for, so what the contact says now wins over
 what the file says.