Preserve CRLF when stripping conflict markers

[oxura]

Fixes #983.

strip_conflict_markers_keep_ours was rebuilding every retained line with \n, so a CRLF working tree silently shifted the byte offsets before attribution merging.

This walks the content with split_inclusive('\n') instead and copies retained slices directly. That keeps CRLF, LF, mixed endings, and the final-newline state exactly as they appeared on disk. It also avoids the temporary Vec and rejoin.

I added a CRLF regression test that produced LF-only output before this change. The focused virtual-attribution tests now pass (13 tests), formatting is clean, and clippy is clean apart from the existing collapsible_match warnings on current main with Rust 1.95.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.