feat(io): add reader implementation for BufRead

[kskalski]

Introduces BufReadAdapter<R> in wincode::io::std_read module, a low-overhead wrapper over any std::io::BufRead source (e.g. BufReader<R>) implementing wincode::io::Reader.

Mix of consuming and scoped borrowing operations supported by checking consume state on each operation and using one of:

• Fast path — when the reader’s internal buffer already holds enough bytes, peek_array and take_scoped return a subslice directly with no allocation, copy_into_slice uses read directly into the dst buffer
• Aux-buffer fallback — when a contiguous slice larger than one currently available from internal reader's fill_buf is requested, bytes are assembled into an aux_buf: Vec<u8>. Subsequent operations can re-serve or extend from this buffer, but switch back to using internal reader once the bytes in aux_buf are consumed.