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).

examples/acp-agent/tests/snapshots/subagent-mixed/input.json

@@ -0,0 +1,8 @@
+{
+  "steps": [
+    { "op": "initialize" },
+    { "op": "newSession" },
+    { "op": "prompt", "text": "Remember this fact for later: the project codeword is SAFFRON. Reply with the single word OK and stop. Do not use any tools." },
+    { "op": "prompt", "text": "Do these two delegations, once at a time. First, use the subagent tool (fresh child) exactly once: 'Reply with exactly the word ALPHA and nothing else.' Then, after it returns, use the subagent_fork tool (forked child that inherits this conversation) exactly once: 'What is the project codeword mentioned earlier in this conversation? Reply with exactly that one word and nothing else.' After both subagents return, reply with the single word PARENT_DONE and stop. Do not use the bash tool." }
+  ]
+}