feat(cli): add diff command to compare two versions

[BryanFRD]

Closes #222

Adds ferrflow diff [package] <from>..<to> — a detailed comparison of what went into a version range.

ferrflow diff v1.4.0..v1.6.0            # single-package repo
ferrflow diff api v1.4.0..v1.6.0        # monorepo: name the package

Output

• Header — <package> <from> → <to> (<overall bump>).
• Commits — every commit in the range with its individual bump (major/minor/patch/none).
• Breaking changes — highlighted separately (any ! / BREAKING CHANGE: commit).
• Files changed — the aggregate file list for the range (capped at 40 with a "… and N more").
• Changelog — the section FerrFlow would generate for the range, using the repo's changelog config.

--json emits the same data as a structured object.

Endpoint resolution

Each side of the range resolves to a commit by trying, in order: the literal string as a tag (a real tag, or v1.4.0 in a single-package repo), then the package's tag for that version (api@v1.4.0). An unresolvable endpoint errors with the candidates it tried.

Implementation

• New version_diff module (the existing diff module is the unified-diff util). Reuses conventional_commits (bump/breaking classification) and build_section_with (changelog rendering) — no new logic duplicated.
• Three small git helpers: get_commits_between / get_changed_files_between (range from..to) and resolve_tag_name_to_commit (peels annotated tags).

Tests

• Unit tests on spec parsing ([package] <range> in any order, range required) and range splitting (rejects empty sides). 973 in the suite, clippy -D warnings clean, wasm builds (the command is CLI-only).
• Smoke on this repo: ferrflow diff v5.40.0..v5.43.0 lists 8 commits with bumps, 33 changed files, and the rendered changelog; --json produces the structured form.

Note

For a monorepo the range spans all commits between the two tags (not path-scoped to the named package) — correct for single-package repos and a useful window audit for monorepos. Path-scoping to the package could be a follow-up.