New marker-based transcript relay/hand-off refusal codes and checks for cross-version updates

Claude Code has an internal relay mechanism, using markers embedded in a transcript, that hands off session state (including prompt cache data) during an /update. It now has specific, named refusal reasons for when this hand-off can't proceed, such as a marker not being in an inactive state, a missing marker line, an unexpected marker line format, or an anchor point not being in view. It also now checks that a "thinking strip" isn't owed, and checks that the resuming session hasn't already answered a turn of its own before arming the relay.