BinaryStream Position does not throw IOException on a closed stream

After closing a BinaryStream, reading the Position property returns a stale value without any error, while reading EndOfFile (or Length) throws an IOException. Behaviour across a closed stream's properties is inconsistent.

Position and its setter now behave like every other closed-stream member: reading or assigning Position after Close() raises an IOException. On an open stream, assigning Position seeks immediately instead of being deferred to the next read or write.