Check a .udc section multiplier against the list the file names

MultWindowType names the list N1MM validates a received section against,
and every exchange that was not all digits counted as a multiplier here.
Four of N1MM's lists are held now -- the ARRL sections, the US states,
the Canadian provinces and the 165 OK/OM districts, which came out of
N1MM's own exe -- and a file naming one of them counts only the values
on it. A file naming any of the hundred-odd others keeps the old rule.

The 2023 OK/OM DX log now scores contact for contact as N1MM did. Two
contacts in the 2026 log still differ: the district code is stored with
a trailing space, which N1MM matches against its list untrimmed, so it
counts no multiplier where this counts one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RoGtneMQaz4M9w7Kk49AVD

src/Nonemm.Contests/Udc/UdcMultipliers.cs

@@ -1,3 +1,4 @@
+using Nonemm.Contests.Multipliers;
 using Nonemm.Core;
 
 namespace Nonemm.Contests.Udc;
@@ -17,6 +18,7 @@ public sealed class UdcMultiplier
     private readonly IReadOnlyList<string> onlyFor;
     private readonly IReadOnlyList<string> notFor;
     private readonly IReadOnlyList<string> onlyForSections;
+    private readonly IReadOnlySet<string>? sections;
 
     public UdcMultiplier(int index, UdcFile file, UdcMultiplierScope shared)
     {
@@ -32,6 +34,7 @@ public sealed class UdcMultiplier
         onlyFor = file.List($"CountMultOnlyFor{suffix}");
         notFor = file.List($"DoNotCountMultOnlyFor{suffix}");
         onlyForSections = file.List($"CountMultOnlyForSec{suffix}");
+        sections = SectionLists.For(file.Text("MultWindowType"));
     }
 
     public bool IsDefined => source.Length > 0;
@@ -127,16 +130,24 @@ public sealed class UdcMultiplier
 
     private static string Upper(string text) => text.Trim().ToUpperInvariant();
 
-    /// A section multiplier ignores a value that is all digits. N1MM checks the
-    /// received exchange against the section list the contest names, and a file
-    /// whose section column also carries the serial numbers the other side of
-    /// the contest sends — OK/OM DX SSB is one — would otherwise count every
-    /// serial number as a multiplier. The section lists themselves are not held
-    /// here.
-    private static string? SectionName(string text)
+    /// A section multiplier counts only a value on the list `MultWindowType`
+    /// names, which is what N1MM checks it against. Without a list — this
+    /// program holds four of N1MM's, and N1MM has over a hundred — anything
+    /// that is not all digits counts, so a file whose section column also
+    /// carries the serial numbers the other side of the contest sends — OK/OM
+    /// DX SSB is one — does not count every serial number as a multiplier.
+    private string? SectionName(string text)
     {
         string section = Upper(text);
-        return section.Length > 0 && section.All(char.IsAsciiDigit) ? null : section;
+        if (section.Length == 0)
+        {
+            return null;
+        }
+        if (sections is not null)
+        {
+            return sections.Contains(section) ? section : null;
+        }
+        return section.All(char.IsAsciiDigit) ? null : section;
     }
 
     private static string? LastLetter(string call)