Fix CRLF line positions after reading records

[onuracar-dev]

Summary

• consume the LF portion of a CRLF terminator before returning a completed record
• preserve the record boundary when CRLF is split across input buffers
• keep reader and record line positions aligned with the next record

Testing

• cargo test --workspace
• cargo build --workspace
• cargo doc --workspace --no-deps
• cargo fmt --all -- --check
• cargo clippy -p csv-core -p csv --lib --tests --examples --bins -- -A clippy::upper_case_acronyms -A clippy::derivable_impls -A clippy::new_without_default -A clippy::wildcard_in_or_patterns -A clippy::result_large_err -A clippy::approx_constant -D warnings

AI assistance disclosure: I used OpenAI Codex to help analyze the parser state machine, implement the change, and run and review the tests. I reviewed the complete diff and validation results.

Fixes #395