Land the commit graph lanes as their own branch

[Defmon3]

Seen on: component/git-history

What it fixes

The plain History tab lists commits without showing how they relate. This piece draws the commit graph: lanes and joins beside each row, so merges and branches are readable at a glance, with the current branch's first-parent line highlighted.

Why we need it

It is PR 5 of the series in the Git History series issue (#8). The graph layout is its own algorithm with its own tests, and reviewing it apart from the list keeps both small.

How to fix it

Cut a fresh branch on top of the plain History tab carrying the graph layout helper, the lane drawing in the commit row, and their tests. The PR description states the end state from the series issue. Nothing deferred.

[Defmon3]

Technical appendix: Land the commit graph lanes as their own branch

Investigated on branch component/git-history at commit 821fc7f, base upstream/main at a6797b3.

Findings

• apps/web/src/lib/gitHistoryGraph.ts (232 lines, new): layoutGitHistoryGraph, which assigns lanes and joins per commit and takes a primary hash set to highlight the first-parent line. gitHistoryGraph.test.ts (new).
• apps/web/src/components/git-history/GitHistoryVisualTypes.ts (25 lines, new): the lane and join types shared by the layout and the row.
• The lane drawing lives inside the row in apps/web/src/components/git-history/GitHistoryCommitList.tsx, which also computes the current head hash and first-parent chain it passes to the layout.
• Size: about 300 source lines plus tests.
• On the stacked proposal branch the layout helper carried a fallback that recomputed the head and first-parent chain when the caller passed none, duplicating the caller's own computation; whether the same fallback exists on this branch was not checked.

The problem

Observed: the graph lanes are inseparable from the commit list today.

Once done: a branch on top of the plain History tab adds lanes and joins to each row, with the current branch's line highlighted, and the layout is covered by its own tests.