Function render_tree 

pub fn render_tree(spans: &[Span], root_id: Option<Uuid>) -> String

Render a span tree as indented text, starting from roots (or the given span).

Output example:

▶ fetch-prices  [started 2026-09-03T12:00:00Z, open]
  ▶ parse-csv   [started ..., ended ...]
    · row-count=42
    · 12:00:01Z  parsed headers
  ▶ insert-db   [started ..., ended ...]