apply_patch rewrites CRLF and mixed line endings to LF on every update

[code-yeongyu]

Summary

Any *** Update File rewrites the whole file with LF line endings. In a CRLF repository, a one-line patch turns into a whole-file change: every line shows as modified in git and in the tool's own diff preview. Files with mixed endings get flattened too.

Reproduction

// lines.txt = "one\r\ntwo\r\nthree\r\n"
await applyPatchDetailed(cwd, "*** Begin Patch\n*** Update File: lines.txt\n@@\n-one\n+ONE\n two\n+between\n three\n*** End Patch\n")
// lines.txt is now "ONE\ntwo\nbetween\nthree\n"

Codex's portable conformance scenarios 023_preserves_crlf_line_endings and 024_preserves_mixed_line_endings (codex-rs/apply-patch/tests/fixtures/scenarios/) fail on this package.

Expected

Only the lines the patch changes are rewritten:

• Unchanged lines, including the chunk's context lines, keep their exact text and ending.
• Inserted lines take the file's first line ending (LF when the file has none).
• The file still ends with a line ending, as today.

This is what Codex implements in codex-rs/apply-patch/src/text_file.rs and file_update.rs (compute_replacements, PreserveLineEndings) at b741e48. In Codex it currently ships behind the apply_patch_preserve_line_endings feature (under development, off by default), and its conformance scenarios run with it on. oh-my-pi restores the file's detected ending after an edit (packages/coding-agent/src/edit/normalize.ts).

Actual

Every line is rewritten with LF.

Acceptance

• Scenarios 023 and 024 pass.
• An LF-only file behaves exactly as before.
• A failed hunk on a CRLF file leaves its bytes untouched.
• The diff preview of a one-line change on a CRLF file counts one line.

[code-yeongyu]

Fixed by #48, merged as d6160ce.

• What changed: an update keeps each line's ending. Unchanged lines and the chunk's context lines keep their exact text and ending, inserted lines take the file's first ending, and LF files produce the same bytes as before.
• Evidence:
  • test/line-endings.test.ts: 6 cases, 4 of them failing on the pre-fix code;
  • full suite 74/74 after rebasing onto fix: reject stray lines between file sections instead of silently dropping them #46, and CI green on abf6c13;
  • Codex's portable scenarios 023 and 024 now pass, so all 25 scenarios pass on main with fix: reject stray lines between file sections instead of silently dropping them #46 and fix: preserve line endings when updating a file #48.