Pick the line terminator for spliced lines through one line_endings::terminator #815

[agent] Filed by the scheduled architecture audit routine (ecosystems and formats). Register: register comment.

Kind: refactor. Source: review 4.4 and 5.4 (CRLF policies); register E16. Child 1 of tracking #814.

Problem

Writers that insert or append lines pick the terminator with three hand-written rules (main @ 045d7ec):

• Any \r\n → CRLF, written 14 times:
  • vendor/common.rs#L157-L163 detect_eol, used by go.sum, go.mod, requirements and yarn classic;
  • vendor/pypi_uv.rs#L1011-L1017 newline_of, a byte-identical private copy;
  • inline if x.contains("\r\n") { "\r\n" } else { "\n" } in formats/gem/hosted.rs#L65, vendor/jvm/maven_reactor.rs#L1519, [`#L1776`](https://github.com/SocketDev/socket-patch/blob/045d7ec783d788bf3c5a1310724b51e09fb6505d/crates/socket-patch-core/src/vendor/jvm/maven_reactor.rs#L1776),`` upstream/composer.rs#L259-L263, [`upstream/gem.rs#L535`](https://github.com/SocketDev/socket-patch/blob/045d7ec783d788bf3c5a1310724b51e09fb6505d/crates/socket-patch-core/src/patch/redirect/upstream/gem.rs#L535),`` upstream/pypi.rs#L629, [`redirect/pipenv.rs#L129`](https://github.com/SocketDev/socket-patch/blob/045d7ec783d788bf3c5a1310724b51e09fb6505d/crates/socket-patch-core/src/patch/redirect/pipenv.rs#L129), [`redirect/npmrc.rs#L614`](https://github.com/SocketDev/socket-patch/blob/045d7ec783d788bf3c5a1310724b51e09fb6505d/crates/socket-patch-core/src/patch/redirect/npmrc.rs#L614), [`redirect/mod.rs#L5775`](https://github.com/SocketDev/socket-patch/blob/045d7ec783d788bf3c5a1310724b51e09fb6505d/crates/socket-patch-core/src/patch/redirect/mod.rs#L5775), [`utils/python_script.rs#L44`](https://github.com/SocketDev/socket-patch/blob/045d7ec783d788bf3c5a1310724b51e09fb6505d/crates/socket-patch-core/src/utils/python_script.rs#L44); plus the `crlf` flags in [`upstream/cargo.rs#L304`](https://github.com/SocketDev/socket-patch/blob/045d7ec783d788bf3c5a1310724b51e09fb6505d/crates/socket-patch-core/src/patch/redirect/upstream/cargo.rs#L304``) and vendor/pypi_pipenv.rs#L153.
• The first line's terminator: vendor/jvm/gradle.rs#L855-L861, a second private newline_of with a different rule under the same name.
• Majority: utils/line_endings.rs#L74-L82 majority_terminator (JsonLayout, composer lock_text).

Drift, proved by execution. A throwaway unit probe on main, run twice, gave:

• for "a\nb\r\nc\r\n": detect_eol and pypi_uv::newline_of → CRLF, gradle::newline_of → LF, majority_terminator → CRLF;
• for "a\r\nb\nc\n": the first two → CRLF, gradle::newline_of → CRLF, majority_terminator → LF.

On a uniform file all three agree.

Symptoms

None filed for these sites specifically. The same disagreement produced #467 and #695 in other writers (see #814).

Impact

Low risk, small. The rules differ only on mixed CRLF/LF files, where a new line comes out in a style that depends on which copy the writer was pasted from.

Proposed change

• Add pub(crate) fn terminator(text: &str) -> &'static str to utils/line_endings.rs: LineEndings::Crlf → "\r\n", Mixed → majority_terminator(text), Lf | None → "\n".
• Replace every site listed above with it.
• Delete: vendor::common::detect_eol, vendor::pypi_uv::newline_of, vendor::jvm::gradle::newline_of, and the 12 inline copies. yarn_classic_lock::block_eol keeps its block-local rule but falls back to terminator (its Yarn classic hosted and vendored rewrites convert LF lines of a mixed CRLF/LF yarn.lock to CRLF, so rollback is not byte-exact #467 behavior is child 3 of Tracking: classify line endings in one place instead of five drifting rules #814).
• Behavior is unchanged for LF-only and CRLF-only files. On a mixed file, inserted lines take the majority style. Majority is stable under appending lines in its own style, so revert paths that remove {line}{nl} (uv [manifest], go.sum, Gradle append_line) still match what the forward pass wrote.

Size and scope

About 15 files, roughly −60/+40 production lines, plus unit tests. Out of scope: preserve_line_endings and crlf_to_lf (child 2), yarn classic's mixed-file posture (child 3, #467), the pnpm CRLF refusal (review 4.7 G) and the Poetry/PDM rule (PR #703).

Acceptance criteria

• rg 'contains\("\\r\\n"\) \{ "\\r\\n"' crates/socket-patch-core/src finds no production hits; detect_eol and both newline_of functions are gone.
• New unit tests in utils/line_endings.rs for terminator: LF, CRLF, None, and mixed with a CRLF majority, an LF majority and a tie.
• A regression test on one append-and-revert writer (for example go_sum_edit or Gradle append_line) over a mixed file: forward then revert is byte-exact.
• These stay green: crlf_go_sum_preserves_line_endings, test_crlf_go_mod_preserves_line_endings, rewrites_hash_pinned_continuation_and_preserves_crlf, transitive_append_crlf_root_without_trailing_newline, crlf_pyproject_and_lock_round_trip_byte_exact, kotlin_settings_crlf_and_missing_newline_are_preserved, crlf_and_tabs_are_preserved (maven_reactor), gem_crlf_lock_checksum_pinned_preserving_crlf, gem_crlf_lock_checksums_header_gains_crlf_entry, gemfile_rerun_on_crlf_checkout_never_nests, and the full cargo test -p socket-patch-core.

Dependencies

Blocked by nothing. It touches gradle.rs and maven_reactor.rs lines that #715's children also edit, so rebase whichever lands second. It unblocks children 2–4 of #814.