fix: show the max width in Git Branch and Git Root Dir previews - #667

What

With a max width set (w), the TUI preview of Git Branch and Git Root Dir now shows the cut, e.g. ⎇ feature... at 12 columns. Before, the preview stayed ⎇ main / my-repo whatever the setting.

Why

w looked like it did nothing in the TUI. Both widgets returned their preview sample before applying the cap. Even if they had applied it, the samples (main, my-repo) were too short for any useful width to cut. The cap only showed on the real status line, with a long branch or repository name.

How

• With a cap set, the preview uses a longer sample (feature/long-branch-name, my-long-repository-name) and cuts it with the same applyMaxWidth the status line uses on a real name. That includes the glyph prefix and the raw value form, so the preview shows what the setting does.
• With no cap, the previews stay main and my-repo.
• Branch link: with a cap set, the preview's sample link points at the sample branch.
• The piped status line doesn't change.

Demo

w on Git Branch set to 12, then on Git Root Dir set to 10. The preview cuts each sample at its limit.

Powerline

Plain

Testing

• New tests: Git Branch's preview cut at 12 columns, labeled and raw (⎇ feature..., feature/l...), and Git Root Dir's at 10 (my-long...). Both failed before the fix. The existing tests that expect ⎇ main and my-repo with no cap still pass.
• The second commit moves Git Branch's preview into its own method, since render() had grown a preview branch with its own sample, width and link handling. compare-widgets.sh renders git-branch the same in all 14 cases on both commits.
• On today's main (with docs: explain an empty status line in one folder (workspace trust) #606 and feat(git-is-fork): render forks as an editable glyph #617 merged in locally): bun test gives 2784 pass, 0 fail (Bun 1.4.2), and bun run lint is clean. Under Node 26 vitest, both widget test files pass (46).
• runtime-check.sh -c max-width-preview on the built CLI: Bun 1.4.2 and Node 26.10.0 agree byte-for-byte, the piped output is the same as main (the change is preview-only), and the TUI opens and exits under both.
• In a real tmux pane: w → 12 on Git Branch turns the preview from ⎇ main into ⎇ feature..., with (max:12) on the row.