Every [Sn] marker resolves to retrieved text
verify.py regexes
[S(\d+)] out of the finished report and
resolves each id against the
EvidenceStore. The id must map to a
registered source that has at least one verbatim snippet behind
it. A source the agent only saw in a search result and never
fetched has no snippet, so citing it fails as
unsupported ; search results are leads,
not evidence.