feat: read fragments collections' Source column

Each row of a fragments collection now names the publication or work
it came from, with acronyms expanded (M.H. -> Mona's Herald); the data
lands in manx-search-data alongside the Date column. The cell is
mapped into DocumentLine.Source and serialized so clients can show
per-line provenance. No indexing or UI changes yet.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: david-allison

CorpusSearch.Test/DocumentLinePreparerTest.cs

@@ -172,6 +172,31 @@ public void LanguageIsReadFromTheManxColumnLanguageCsvColumn()
         }
     }
 
+    /// <summary>The CSV contract of a fragments collection: each row carries its
+    /// Date cell (read into the line date) and its Source cell (the publication,
+    /// acronyms expanded)</summary>
+    [Test]
+    public void DateAndSourceAreReadFromTheirCsvColumns()
+    {
+        var path = Path.GetTempFileName();
+        try
+        {
+            File.WriteAllText(path,
+                "English,Manx,Notes,Date,Source\n" +
+                "parson,saggyrt,\"[1] IoME, Sat, Sep 21, 1901; Page: 3\",21/09/1901,Isle of Man Examiner\n" +
+                "honey,mill,,21/09/1901,Isle of Man Examiner\n");
+
+            var lines = CsvHelperUtils.LoadCsv(path);
+
+            Assert.That(lines.Select(x => x.DateCell), Is.All.EqualTo("21/09/1901"));
+            Assert.That(lines.Select(x => x.Source), Is.All.EqualTo("Isle of Man Examiner"));
+        }
+        finally
+        {
+            File.Delete(path);
+        }
+    }
+
     /// <summary>[MS 1 Thessalonians 2.16] Text... - the citation becomes line
     /// metadata and leaves the Manx token stream</summary>
     [Test]

CorpusSearch/Dependencies/CsvHelper/DocumentLineMap.cs

@@ -19,6 +19,7 @@ public DocumentLineMap()
         // "Language" header would not say which column it describes
         Map(m => m.Language).Name("ManxColumnLanguage").Optional();
         Map(m => m.DateCell).Name("Date").Optional();
+        Map(m => m.Source).Optional();
         Map(m => m.CsvLineNumber).Convert(args => args.Row.Parser.RawRow);
         Map(m => m.ManxOriginal).Convert(args =>
         {

CorpusSearch/Model/DocumentLine.cs

@@ -68,6 +68,11 @@ public class DocumentLine
     [JsonIgnore]
     public string? DateCell { get; set; }
 
+    /// <summary>The publication or work the row's fragment came from ("Mona's
+    /// Herald"), acronyms expanded: the `Source` cell of a fragments collection.
+    /// Serialized, so a client can show per-line provenance. Nullable.</summary>
+    public string? Source { get; set; }
+
     /// <summary>The line's own date, where it has one apart from its document's: in
     /// a fragments collection (<see cref="NotesCitationDates"/>) each line dates
     /// from its Date cell, or - in data predating the column - from its note's