internal/core/frontmatter has no entry in the internal README package map

internal/core/frontmatter has no entry in the internal README package map

Surfaced by the itd-179 round-3 builder as an observation and captured rather than fixed: pre-existing, and unrelated to the round's records.

internal/core/frontmatter carries no row in internal/README.md's package map. The gap matters slightly more after round 3 than before it, because that package is now the canonical home of Unquote — the decoder consolidated out of capture.unquote and lint.unescapeScalar — so a reader looking for the one true scalar decoder has no map entry pointing there.

Grounds

• pursued: a reader looking for the one scalar decoder finds it from the package map; a second decoder appearing outside the package would show it wrong