Fix OldSpec byte[] encoding emitting unsupported str8 headers - #2287

With MessagePackWriter.OldSpec = true, writing a byte[] of length 32–255 incorrectly emitted the post-2013 str8 marker (0xD9), which legacy MessagePack readers cannot decode.

Cause

WriteBinHeader correctly delegated to WriteStringHeader in old-spec mode, but WriteStringHeader always used MessagePackPrimitives.TryWriteStringHeader, which selects str8 for those lengths. The string write path already avoided str8 via WriteString_PostEncoding; header writes did not.

Changes

• WriteStringHeader: when OldSpec is set and length is 32–255, emit str16 (0xDA) instead of str8
• Docs: correct OldSpec remarks that listed Str8 as a valid old-spec encoding
• Tests: cover writer/byte[] paths at the 31/32/255/256 boundaries and extend OldSpecBinaryFormatterTest with 32 and 255

Example

byte[] value = new byte[32];
Array.Fill(value, (byte)'A');
ArrayBufferWriter<byte> buffer = new();
MessagePackWriter writer = new(buffer) { OldSpec = true };
writer.Write(value);
writer.Flush();
// before: D92041...  (str8)
// after:  DA002041... (str16)

• Fixes MessagePackWriter.OldSpec is ignored for byte[] values in v3 #2286