compact: add FullWithBoundary for full transcript + scope marker

FullWithBoundary compacts the entire transcript (ignoring StartLine for truncation) and returns the full compact bytes plus the line index at which this checkpoint's slice begins in the compact output. The boundary is derived as lineCount(full) - lineCount(delta), reusing each format's existing, independently-tested slicing behavior rather than threading source indices through all seven emitters. The only imprecision is a deterministic off-by-one when a single logical message straddles the exact boundary.

This is the foundation for storing a full, self-contained compact transcript on every checkpoint (so a session is reconstructable from any single checkpoint) while still letting readers segment a checkpoint's slice via the marker.