Fix Musepack SV8 Stream Header parsing

The parser was only skipping 1 byte before reading sample count, but
per Musepack SV8 spec the Stream Header payload starts with:
- 4 bytes: CRC32
- 1 byte: Stream version
- Variable length: Sample count
- Variable length: Beginning silence
- 1 byte: Flags

Updated parser to skip 5 bytes (CRC + version) before reading the
variable-length sample count. Also fixed the local test helper to
include the 4-byte CRC placeholder for consistency with TestBuilders.

Author: decriptor

src/TagLibSharp2/Musepack/MusepackFile.cs

@@ -284,9 +284,11 @@ private static void ParseSV8StreamHeader (ReadOnlySpan<byte> payload, MusepackFi
 
 		var offset = 0;
 
-		// Skip CRC/version byte
-		if (offset >= payload.Length) return;
-		offset++;
+		// Skip CRC32 (4 bytes) + Stream version (1 byte)
+		// Per Musepack SV8 spec, SH payload starts with these 5 bytes
+		if (payload.Length < 5)
+			return;
+		offset += 5;
 
 		// Sample count (variable-length integer)
 		var sampleCountResult = ReadVarInt (payload, offset);

tests/TagLibSharp2.Tests/Musepack/MusepackFileTests.cs

@@ -746,7 +746,9 @@ private static byte[] CreateSV8StreamHeaderPacket (int sampleRate, int channels,
 		using var payloadMs = new MemoryStream ();
 
 		// SH payload:
-		// [0] CRC (skip for test - 1 byte for version indicator)
+		// [0-3] CRC32 placeholder (4 bytes)
+		payloadMs.Write (new byte[4]);
+		// [4] Stream version
 		payloadMs.WriteByte (8); // Version 8
 
 		// Sample count as variable-length integer