An overflow panic in Reader::read_byte_record

### Summary
`csv` v1.4.0 can panic in debug builds due to unchecked addition in the `read_byte_record` call path:

- `src/reader.rs:1649`
- `set_byte(byte + nin as u64)`

If `byte == u64::MAX` and `nin > 0`, this overflows and panics.

### Why this is report-worthy
- This panic is reachable from public APIs (`Reader`, `Position`, `seek_raw`, `read_byte_record`).
- The function docs do not mention this panic condition.
- The operation currently uses unchecked `+` instead of `checked_add`/error return.

### Public API reproducer

```rust
use std::io::{Cursor, SeekFrom};
use csv::{ByteRecord, Position, Reader};

#[test]
#[should_panic]
fn panic_arithmetic_overflow_read_byte_record_impl_line_1649() {
let mut rdr = Reader::from_reader(Cursor::new(b"h\nx\n".to_vec()));

let mut pos = Position::new();
pos.set_byte(u64::MAX);

rdr.seek_raw(SeekFrom::Start(0), pos).unwrap();

let mut record = ByteRecord::new();
let _ = rdr.read_byte_record(&mut record);
}
```
### Call chain
1. Reader::from_reader(...)
2. Reader::seek_raw(..., pos_with_byte_u64_max)
3. Reader::read_byte_record(...)
4. Internal read_byte_record_impl(...)
5. set_byte(byte + nin as u64) at src/reader.rs:1649 panics when nin > 0

### Actual behavior
Panic on integer overflow (debug builds).

### Expected behavior
Either:

+ avoid panic via checked arithmetic and return an error, or
+ explicitly document panic preconditions in API docs.
### Suggested fix
At src/reader.rs:1649, replace unchecked addition with checked handling, e.g.:

+ byte.checked_add(nin as u64) and map overflow to Error,
+ or saturating behavior if that matches crate semantics.
### Version
+ crate: csv
+ version: 1.4.0