Fix mixed EOL post-commit attribution

[zhangzimingmmz]

Summary

• Normalize line terminators during post-commit virtual attribution comparisons
• Avoid treating CRLF-vs-LF-only differences as unstaged/carryover changes
• Add mixed-EOL regression coverage for Windows/autocrlf workflows

Tests

• cargo test carryover_merge_ignores_mixed_eol_when_content_matches_committed
• cargo test --test integration test_post_commit_preserves_ai_attribution_for_mixed_eol_worktree
• cargo fmt -- --check
• cargo clippy --all-targets -- -D warnings
• cargo test -- --test-threads 12 (Docker rust:1.93.0, non-root)

[CLAassistant]

All committers have signed the CLA.