The seed-boundary change made fork-child replay route correctly but shipped
with no recorded fork scenario — the seedLength slice was exercised only by
llm-replay unit tests and a persistence round-trip, never by the full-transcript
snapshot tier. Add two recorded scenarios that drive a real fork child through
it:
- subagent-fork: parent completes a turn, then forks one child (child fixture
carries a non-zero seedLength, the boundary the replay slice consumes).
- subagent-mixed: parent completes a turn, then delegates once via spawn
(seedLength 0) and once via fork (non-zero seedLength) in one transcript —
the first scenario to drive two subagent backends at once, exercising both
branches of the slice.
Both need a completed turn-1 so the fork seed is a non-empty completed-turn
prefix (a turn-1 fork seeds empty = spawn, which would not exercise the slice).
Removing the slice turns both scenarios red (the fork child receives the
parent's recorded chunks), proving the guard bites.
ACP (out-of-process) subagent replay remains a different shape, still tracked
as TODO(acp-subagent-replay).
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