fix(ui): scroll branch tree selections

[abhinav]

Implement scrolling for branch selection prompts
when the terminal cannot show all matching branches.

Add viewport support to fliptree
as the renderer that owns line-by-line tree output,
including scroll marker rendering.
Keep branchtree as the branch-specific adapter
that passes viewport settings through,
and keep BranchTreeSelect responsible for
selection state, filter matching,
and choosing the viewport offset.

Extend the UI script harness
to accept per-script terminal sizes,
and add regression coverage for
constrained-height scrolling
and preserving an already selected branch
when filtering still matches it.

[abhinav]

This change is part of the following stack:

• fix(ui): scroll branch tree selections #1058 ◀

_{Change managed by git-spice.}