Centralize ZIP filename lookup

Add FindFileIndex to resolve the first case-insensitive filename match
without extracting it. Reuse the lookup in named extraction and library
include resolution, removing the duplicate archive-entry scan while
preserving archive order and source fallback behavior.

Validated with a warning-free Qt 6.10 Debug build and 51 offscreen
lifecycle groups; the final API rename also builds without warnings.

common/lc_library.cpp

@@ -1809,20 +1809,17 @@ lcResult<std::unique_ptr<lcFile>> lcPiecesLibrary::OpenPieceDataFile(const char*
 				{
 					const QByteArray FileName = (Prefix + Folder + QString::fromLatin1(PieceName)).toLatin1();
 
-					for (quint32 Index = 0; Index < Archive->mFiles.size(); Index++)
-					{
-						if (qstricmp(Archive->mFiles[Index].file_name, FileName.constData()))
-							continue;
+					const std::optional<quint32> FileIndex = Archive->FindFileIndex(FileName.constData());
 
-						lcFileSource Source;
-						Source.ArchiveType = static_cast<int>(Type);
-						Source.ArchiveIndex = Index;
+					if (!FileIndex)
+						continue;
 
-						if (lcResult<std::unique_ptr<lcFile>> File = OpenFile(Source))
-							return File;
+					lcFileSource Source;
+					Source.ArchiveType = static_cast<int>(Type);
+					Source.ArchiveIndex = *FileIndex;
 
-						break;
-					}
+					if (lcResult<std::unique_ptr<lcFile>> File = OpenFile(Source))
+						return File;
 				}
 			}
 

common/lc_zipfile.cpp

@@ -567,17 +567,24 @@ bool lcZipFile::ReadCentralDir()
 	return true;
 }
 
-bool lcZipFile::ExtractFile(const char* FileName, lcMemFile& File, quint32 MaxLength)
+std::optional<quint32> lcZipFile::FindFileIndex(const char* FileName) const
 {
 	for (quint32 FileIdx = 0; FileIdx < mFiles.size(); FileIdx++)
 	{
-		lcZipFileInfo& FileInfo = mFiles[FileIdx];
+		const lcZipFileInfo& FileInfo = mFiles[FileIdx];
 
 		if (!qstricmp(FileInfo.file_name, FileName))
-			return ExtractFile(FileIdx, File, MaxLength);
+			return FileIdx;
 	}
 
-	return false;
+	return std::nullopt;
+}
+
+bool lcZipFile::ExtractFile(const char* FileName, lcMemFile& File, quint32 MaxLength)
+{
+	const std::optional<quint32> FileIndex = FindFileIndex(FileName);
+
+	return FileIndex && ExtractFile(*FileIndex, File, MaxLength);
 }
 
 bool lcZipFile::ExtractFile(quint32 FileIndex, lcMemFile& File, quint32 MaxLength)

common/lc_zipfile.h

@@ -59,6 +59,9 @@ public:
 	bool OpenRead(std::unique_ptr<lcFile> File);
 	bool OpenWrite(const QString& FileName);
 
+	// Returns the first case-insensitive filename match in archive order.
+	std::optional<quint32> FindFileIndex(const char* FileName) const;
+
 	bool ExtractFile(quint32 FileIndex, lcMemFile& File, quint32 MaxLength = 0xffffffff);
 	bool ExtractFile(const char* FileName, lcMemFile& File, quint32 MaxLength = 0xffffffff);