PM decision B6 asked for a list-taking _render_sources so a concept can record
more than one source, and prescribed the block list as the emitted form. The
list is delivered; the block form is not.
Three measurements, not an argument. Our own parse_frontmatter skips indented
lines, so a block list round-trips to an empty value with every entry silently
gone -- and _is_ingest_owned reads through that same parser. The consumer B6
was written for accepts the multi-entry flow sequence and classifies a block
sequence as unreadable provenance, so block would hand it exactly the state it
cannot read. And B6's own acceptance test asks for a round trip through this
parser, which no block form can pass.
A single source renders byte-identically, so all six goldens are unmoved. The
unquotable-value gate now runs on every entry, not just the first. New code
sources_empty refuses an empty list.
1023 -> 1034 tests, including the negative control that pins the block form's
silent data loss.
