Function changelog_sections 

pub fn changelog_sections(
    project_root: &Path,
    range_start: &str,
) -> Result<Vec<(ChangelogHeading, Vec<String>)>, VersionError>

Group --no-merges commits in range_start..HEAD by ChangelogHeading (D-12). Walks the identical range and git log --no-merges <range> --format=%H%x1f%B%x1e argv as classify_range_bump (same record separators, same git_conventional::Commit::parse call) — but, unlike classify_range_bump (which folds every commit down to a single aggregate Bump value; see RESEARCH.md Pitfall 1), collects each commit’s subject into its group instead of discarding it. classify_range_bump’s returned Bump is never used as changelog content; this is sibling code, not a wrapper around it.

Complete per-type mapping (D-12, Task 2), evaluated in this order:

1. git_conventional::Commit::parse fails → ChangelogHeading::Changed, bullet = the message’s first line.
2. commit.breaking() is true → ChangelogHeading::Breaking — checked before the type match, mirroring classify_commit_message’s own precedence.
3. type is feat → ChangelogHeading::Added.
4. type is fix/perf → ChangelogHeading::Fixed.
5. every other type (docs, test, chore, "ci", refactor, style, or any other recognized-but-unlisted type) → ChangelogHeading::Changed.

Deliberate divergence from classify_commit_message: an unparseable message is Bump::Patch for versioning (D-10’s floor — an unrecognized commit still bumps something) but Changed here — a message with no conventional type has no claim to Fixed. Do not “fix” this into agreement; it is intentional.

Bullets preserve git-log order (newest first) within each group; groups are emitted in ChangelogHeading declaration order, omitting any group with no bullets. A range with no commits returns Ok(Vec::new()).