Align follow-up composer stack carets

[brsbl]

What was wrong

The three right-edge disclosure controls in the follow-up composer stack did not share a horizontal geometry contract. The branch and queued-message controls centered their 16px chevrons in a 24px edge column, while the composer collapse control used a 32px icon button against the same right edge. That put the composer caret four pixels left of the carets above it.

What changed

• Adds one shared 24px horizontal edge-caret class for the follow-up prompt stack.
• Applies it to the branch banner, queued-message header, and wide composer collapse control while preserving their existing vertical hit targets.
• Adds a regression assertion at the stable composer behavior boundary so the collapse control cannot drift back to a wider horizontal box.
• No wire, persistence, SDK, CLI, or daemon protocol behavior changed.

How you verified

• Red proof: with the production composer class absent, the new assertion fails because the collapse control does not contain w-6.
• Green proof: the focused composer assertion passes, and neighboring queued-message coverage passes 36/36.
• Exact rebased head 400f7c6674cbeb5a1df9c2f331f97b020865c3ba passes every required GitHub check, including all app shards, packages, server, integration, and Linux/macOS package smoke.
• Chrome for Testing 152.0.7977.64 rendered the same Stacked cards with pills fixture at 1440×900 on exact parent and child heads. On the child, branch, queue, and composer caret centers all measured x=1019; the deliberate real-app pass also covered expanded/collapsed behavior, plus/model menus, focus re-expansion, responsive overflow, and runtime exceptions. The later rebase changed only ancestor SDK inventory metadata, not this layer's app tree. Safari is not required for this non-marketing bb UI change.

Before — parent head f4707e0ad2ea6622fa4c314e943c6735a1d21f34

The composer caret sits four pixels left of the banner and queue carets.

After — child head 400f7c6674cbeb5a1df9c2f331f97b020865c3ba

All three carets share the same horizontal center.

BB-Thread-ID: thr_sjdd7gudiq

AGENT GENERATED