feat(core): replace instruction checkpoints with value-delta sync (#36254)

packages/core/src/session.ts

@@ -194,7 +194,7 @@ export interface Interface {
    */
   readonly pending: (sessionID: SessionSchema.ID) => Effect.Effect<SessionPending.Info[], NotFoundError>
   /**
-   * Durable, ordered, gap-free session log read. Replays public durable
+   * Durable, ordered session log read. Replays public durable
    * session events after the exclusive `after` cursor, emits a `Synced`
    * marker at the captured replay watermark, then continues live when `follow`
    * is set.
@@ -384,9 +384,11 @@ const layer = Layer.effect(
         if (input.messageID && !boundary)
           return yield* new MessageNotFoundError({ sessionID: input.sessionID, messageID: input.messageID })
         const sessionID = SessionSchema.ID.create()
+        const parentSeq = boundary ? boundary.seq - 1 : yield* EventV2.latestSequence(db, parent.id)
         yield* events.publish(SessionEvent.Forked, {
           sessionID,
           parentID: parent.id,
+          parentSeq,
           from: input.messageID,
         })
         return yield* result.get(sessionID).pipe(Effect.orDie)