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/support/llm-replay/tests/llm-replay.spec.ts

@@ -35,12 +35,13 @@ const TEXT_CHUNKS: StreamChunk[] = [
 ]
 
 /** Build a minimal session-JSONL string: a header line + the given events. */
-function sessionJsonl(events: SessionEvent[], header?: { id?: string; createdAt?: number }): string {
+function sessionJsonl(events: SessionEvent[], header?: { id?: string; createdAt?: number; seedLength?: number }): string {
   const headerLine = JSON.stringify({
     type: 'session',
     version: 0,
     id: header?.id ?? 's1',
     createdAt: header?.createdAt ?? 0,
+    ...header?.seedLength !== undefined ? { seedLength: header.seedLength } : {},
   })
   return [headerLine, ...events.map(e => JSON.stringify(e))].join('\n') + '\n'
 }
@@ -370,17 +371,22 @@ describe('installLlmReplay (through the real waterfall)', () => {
 })
 
 describe('parseSessionHeader', () => {
-  it('reads id and createdAt off the header line', () => {
+  it('reads id, createdAt, and seedLength off the header line', () => {
     expect(parseSessionHeader(sessionJsonl([], { id: 'abc', createdAt: 42 })))
-      .toEqual({ id: 'abc', createdAt: 42 })
+      .toEqual({ id: 'abc', createdAt: 42, seedLength: 0 })
   })
 
-  it('falls back to id="" / createdAt=0 when the header lacks them', () => {
-    expect(parseSessionHeader('{"type":"session","version":0}\n')).toEqual({ id: '', createdAt: 0 })
+  it('reads a non-zero seedLength (a fork child header)', () => {
+    expect(parseSessionHeader('{"type":"session","version":0,"id":"child","createdAt":7,"seedLength":4}\n'))
+      .toEqual({ id: 'child', createdAt: 7, seedLength: 4 })
+  })
+
+  it('falls back to id="" / createdAt=0 / seedLength=0 when the header lacks them', () => {
+    expect(parseSessionHeader('{"type":"session","version":0}\n')).toEqual({ id: '', createdAt: 0, seedLength: 0 })
   })
 
   it('falls back on an empty buffer (no header line)', () => {
-    expect(parseSessionHeader('')).toEqual({ id: '', createdAt: 0 })
+    expect(parseSessionHeader('')).toEqual({ id: '', createdAt: 0, seedLength: 0 })
   })
 })
 
@@ -420,6 +426,32 @@ describe('loadSessionScripts', () => {
       .toThrow(/child fixture not found/)
   })
 
+  it('derives a FORK child script from its OWN events only (skips the seeded parent prefix)', () => {
+    // A fork child's log begins with the seeded parent prefix — the parent's
+    // events, INCLUDING its assistant/chunk events. Deriving the child script
+    // from the whole log would replay the PARENT's recorded responses as the
+    // child's model calls. With seedLength recorded, the child script must
+    // contain only the child's OWN chunks (those after the boundary).
+    const parentChunk: StreamChunk = { type: 'text-delta', index: 0, text: 'PARENT-RESPONSE' }
+    const childChunks: StreamChunk[] = [{ type: 'text-delta', index: 0, text: 'CHILD-RESPONSE' }, { type: 'finish', reason: { kind: 'stop' } }]
+    const f = writeSession('session.jsonl', { id: 'parent', createdAt: 100 }, [TEXT_CHUNKS])
+    // The child fixture: 2 seeded parent events (a chunk + its finish) then the
+    // child's own turn. seedLength = 2 marks where the inherited prefix ends.
+    const childEvents: SessionEvent[] = [
+      chunkEvent(0, 1, 1, parentChunk),
+      chunkEvent(1, 1, 1, { type: 'finish', reason: { kind: 'stop' } }),
+      chunkEvent(2, 2, 1, childChunks[0]!),
+      chunkEvent(3, 2, 1, childChunks[1]!),
+    ]
+    const childPath = join(dir, 'session.1.jsonl')
+    writeFileSync(childPath, sessionJsonl(childEvents, { id: 'child', createdAt: 200, seedLength: 2 }), 'utf8')
+
+    const scripts = loadSessionScripts({ file: f, childFiles: [childPath] })
+    // The child script is ONLY the child's own model call — the parent's seeded
+    // chunk is gone.
+    expect(scripts[1]?.entries).toEqual([{ kind: 'chunks', chunks: childChunks }])
+  })
+
   it('uses the override for the primary and still derives children', () => {
     writeFileSync(file, sessionJsonl([], { id: 'p', createdAt: 1 }), 'utf8')
     const overrideFile = join(dir, 'replay.override.json')