Motivation

• PEM corruption helpers normalized all outputs to LF which broke expectations for inputs using CRLF line endings; deterministic negative fixtures must preserve input line separators.
• Add focused edge-case coverage so injected corruptions and header/footer replacements behave consistently for CRLF and LF inputs.

Description

• Refactor PEM corruption assembly to be separator-aware by adding line_separator, join_lines, and push_line helpers and using them in replace_first_line, replace_last_line, inject_bad_base64_line, and inject_blank_line in crates/uselesskey-core/src/srp/negative/pem.rs.
• Update early-return behavior for short inputs to append the correct separator rather than hardcoding \n.
• Add unit tests that assert CRLF-preservation for header replacement, bad-base64 injection, blank-line injection, and short malformed PEM inputs.
• Keep existing LF-based behavior intact for backwards compatibility and test stability.

Testing

• Ran cargo test -p uselesskey-core srp::negative::pem and the PEM-focused test suite passed (all tests OK).
• Ran cargo test -p uselesskey-core and core crate tests passed with no failures.
• Ran cargo xtask lint-fix --check (formatting and clippy) which completed successfully.

Codex Task