Builds on #13544
This adds a new CONTINUATION record type that is sent before READY.
CONTINUATION contains the bytes (if any) that will bring a ground-state
VT state machine up to the same state.
This allows snapshotting a terminal instance that is, for example,
blocked waiting for a caller to complet an in-flight Kitty graphics
protocol send. In practice, I think this will be rare. But in theory, it
avoids a DoS-type attack.
The continuation state must be the MINIMAL set of bytes that will move
the virtual terminal state from a ground to non-ground state. The reason
it must be minimal is because any extra bytes can duplicate work into the
terminal that might already exist.
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