Persist the seed boundary so fork-child replay routes correctly

A fork subagent seeds its child session with a prefix of the parent's log, and
that seed becomes the child's persisted log — so a fork child's .jsonl begins
with the PARENT's events, including the parent's assistant/chunk events. The
snapshot replay harness derived a child's script from its whole log, which would
replay the parent's recorded responses as the child's model calls. Spawn-only
scenarios never hit it, but a fork snapshot would mis-route silently.

Record the seed boundary and skip the inherited prefix at replay:

- SessionHeader gains an optional `seedLength` (how many leading events were
  inherited via a seed), threaded through CreateSessionOptions/CreateAgentOptions
  meta and stamped by the fork backend (= seeded-prefix length; absent for spawn).
  It is EXPLICIT, never inferred from seed.length: a resume seeds the whole stored
  log, so the resume path passes the persisted boundary back.
- Both persistence backends round-trip it: JSONL header line, SQLite seed_length
  column. The SQLite table change bumps SCHEMA_VERSION 2->3; per the pre-release
  stance the backend rejects an older user_version on open with NO migration.
- llm-replay's parseSessionHeader reads seedLength and loadSessionScripts derives
  a child script from events AFTER the boundary. seedLength is 0 for spawn, so
  spawn replay is byte-for-byte unchanged.

Closes the routing-correctness gap the per-session snapshot replay RFC under-
stated; a recorded fork scenario remains a future addition but now derives
correctly. RFC: docs/rfc/implemented/testing/2026-06-22-fork-child-replay-seed-boundary.md.

Regression coverage: a fork child fixture whose seeded prefix carries a parent
chunk (derived script must exclude it, proven red without the slice); a seedLength
persistence round-trip through the shared coordinator contract (both backends);
the fork backend stamping it; resume preserving it from the persisted header.

packages/session-persistence/session-persistence/tests/coordinator-contract.ts

@@ -123,6 +123,26 @@ export function runCoordinatorContract(name: string, makeFixture: () => Promise<
       }
     })
 
+    it('round-trips the seed boundary (seedLength) through persistence', async () => {
+      // A forked child records how many leading events were inherited via the
+      // seed; the boundary must survive a reload (so a resume/replay can tell the
+      // inherited prefix from the child's own events). Both backends carry it on
+      // the header — JSONL on the header line, SQLite in the seed_length column.
+      const fix = await makeFixture()
+      const { ctx, fiber } = await freshCtx(fix)
+      try {
+        const session = ctx.sessions.create(SessionId('forked-child'), { meta: { cwd: WORK, seedLength: 3 } })
+        send(session, oneTurnLog())
+        await ctx.parallel('session/flush', session)
+
+        const loaded = await ctx.sessionPersistence.load(SessionId('forked-child'))
+        expect(loaded.meta.seedLength).toBe(3)
+      } finally {
+        await fiber.dispose()
+        await fix.cleanup()
+      }
+    })
+
     it('snapshot-on-buffer: mutating an event after session/event does not corrupt the persisted copy', async () => {
       const fix = await makeFixture()
       const { ctx, fiber } = await freshCtx(fix)