`thiserror`-derived `Display` does not walk `#[source]`, so `format!("{err}")`
and `format!("{err:#}")` on a typed error silently produce only the
top-level message — the same format string changes meaning when migrating
from `anyhow::Result` to a typed `Result`. Point at
`fabro_util::error::collect_chain` as the canonical helper and broaden
the test guidance to cover typed errors.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>