JS-032: `bin/lib/dag.js` — ahead/behind commit divergence

Now: DONE

Add the commit-divergence block (ahead/behind) to bin/status.js via a JS DAG. Part of JS-027. Method: Issues.

Input

Context

be status prepends ahead/behind cur-vs-tip (GET-021; graf in C). graf is a dog, so reimplement the ancestry in JS over JS-030 commitParents (bounded BFS).

Goals

dag.js: aheadBehind(cur, tip) -> {ahead, behind} + the diverging commit:?sha list, via parent-walk over keeper commits.
Wire the commit-diff block into bin/status.js ahead of the file rows.

Constraints

libdog+abc ONLY (no graf linked). Bounded walk; visited set in JS.

WIP

Design decisions

v1 first-parent + merge parents; foster/rebase parents excluded from the FF walk (match git/graf per dog/git/GIT.h).

TODOs

Repro-first (test/bin/): ahead/behind + the divergence rows match native be status on a diverged toy repo (local commit + advanced tip).

Blockers

← JS-030.

Outcome

(pending)