fix(opy): support continued preprocessing directives

[e54-bot]

Summary

• Keep backslash-newline continuations inside #! directives as one directive body.
• Preserve the existing cross-line span tracking and add lexer regression coverage.
• Update the real-world corpus expectations to the next preprocessing boundary after the lexical gap is cleared.

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-targets --all-features
• python3 -m unittest discover -s compatibility/tests
• Full native compatibility corpus: 61 fixtures passed

Fixes #141

[Teakowa]

LGTM