Fork ("Fork from here") on a large/compacted session could cut context off a
real turn boundary — slicing mid-turn or leaving a dangling tool call, feeding
a malformed context to the model. Align the fork cut to a resolved turn boundary
via a signature matcher handling both compacted and non-compacted display
coordinate spaces; errs toward under-keeping with send-time sanitization backstop.
Co-authored-by: b3nw <b3nw@users.noreply.github.com>
