tui: playbook collaborator label paints over the document line above the caret

[edwin-zvs]

A collaborator's label is drawn one row above their caret, at the caret's column, straight over whatever is already rendered there:

let rect = Rect::new(pos.x.saturating_add(1), pos.y - 1, label_w, 1);
f.render_widget(Paragraph::new(label).style(label_style), rect);

(crates/cli/src/ui.rs ~lines 16962–16968 @ ad47152)

There is no check for occupied cells — unlike the neighbouring agent-reveal tint, which is explicitly written to leave already-styled cells alone.

Repro

1. Open the same session's Playbook in the TUI and the web UI.
2. In the web UI, put the caret in the middle of a line that has a non-empty line above it.

Document text (TUI, 180 cols):

  • build the thing ZZZ
  • verify it!

Rendered while the web caret sits on verify it!:

  • build tWeb 1ing ZZZ
  • verify it!

Web 1 has replaced he th. The user's own document appears corrupted; there is no way to tell the badge from real text in a screenshot or a copied capture.

The web UI does this correctly — the peer's TUI 1 flag floats above the line without displacing characters.

Expected

The label should not destroy document glyphs. Options:

• render it in the right margin / gutter of the caret's row,
• render it only when the target cells are blank, falling back to a caret-colour tint plus a hover/status affordance,
• reserve a one-row gap the way the web UI's floating flag does.

Acceptance criteria

• A collaborator label never overwrites document text in the Playbook body.
• The collaborator is still identifiable (label, or colour + another affordance) when the row above is occupied.
• Test covering a peer caret on a line whose predecessor is non-blank.

Found during a hands-on UX audit of the Playbook in the TUI and web UI.