TranscriptTailState.profile was documented as the owning profile but now
holds the request ROUTE (the owner is the map key). Rename the
recordTranscriptTail parameters to route/owner, fix the field doc, say why
a bare-string scope still defaults to 'default' while an object scope does
not, and correct transcript-tail-cache's claim that it mirrors the
in-memory key (it keys by request scope and the two stores never cross-read).
fd9c52ae1e