Record fork and mixed spawn+fork snapshot scenarios

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).

docs/rfc/README.md

@@ -143,6 +143,7 @@ Do NOT write one for a mechanical or local choice (a variable name, a one-file r
 | [Use `session.jsonl` as the only snapshot session-log artifact](implemented/testing/2026-06-20-remove-redundant-snapshot-log-goldens.md) | 2026-06-20 |
 | [Per-session snapshot replay for nested agents](implemented/testing/2026-06-22-subagent-snapshot-replay.md) | 2026-06-22 |
 | [Persist the seed boundary so fork-child replay routes correctly](implemented/testing/2026-06-22-fork-child-replay-seed-boundary.md) | 2026-06-22 |
+| [Record fork and mixed spawn+fork snapshot scenarios](implemented/testing/2026-06-22-fork-snapshot-scenarios.md) | 2026-06-22 |
 
 ## Rejected