Marker prefix appended to every v2 review-comment body. Mirrored by
crate::output_envelope::MARKER_REGEX_V2; both must change together
because consumers extract the fingerprint by running the regex over a
body whose marker line uses this prefix. The :v2: namespace prevents
collision with v1 historical markers and reduces user-paste spoofing
risk (typing :v2: by accident is unlikely).
Render path the print site uses. Exposed so unit tests can pass a
hand-crafted DiffIndex without poking the process-wide SHARED_DIFF
cache (which is OnceLock-bounded and not reentrant under cargo test’s
parallel runner).