fix(agent-loop): open turns before pre-step

.agents/notes/implemented/simplification/2026-07-17-one-send-one-turn.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write .agents/notes/implemented/simplification/2026-07-17-one-send-one-turn.md
-2026-07-17-one-send-one-turn.md: 6d2e9f57caf73754ea8c8d9e711126de2e7c885c
-2026-07-17-one-send-one-turn.zh.md: 1d2e74bbcf9478020cf0dfcf7ebdbb2349a86022
+2026-07-17-one-send-one-turn.md: 834eb2b3aace0a0aa5f86f483e7b7d7aabbe467e
+2026-07-17-one-send-one-turn.zh.md: aff3164d58dde238863a60fbcdbab6dab2d09961

.agents/notes/implemented/simplification/2026-07-17-one-send-one-turn.md

@@ -20,7 +20,7 @@ Before inserting a message, `send()` checks the agent state and accepts an alrea
 
 If messages A and B are both processed, B's turn starts only after A records `turn/end` and A's durability checkpoint settles. B's request therefore sees whatever closed result A left in the same session log. A checkpoint error is reported, but settlement only releases this ordering barrier; it does not make a failed write durable. Broad `cancel()`, disposal, or a failure before `turn/start` can instead discard an unstarted item without opening an empty turn.
 
-At a turn boundary, the loop claims one follow-up after pending next-step input. `agent/pre-step` either rejects the proposal or returns the complete entering batch. A rejected follow-up remains removed without opening a turn or writing session history. Mixed ordinary follow-up branches do not exist.
+At a turn boundary, the loop opens the turn and claims one follow-up after pending next-step input. `agent/pre-step` either rejects the proposal or returns the complete entering batch. A rejected follow-up remains removed and closes a blocked no-step turn without writing model-visible history. Mixed ordinary follow-up branches do not exist.
 
 The no-batching rule applies only to ordinary follow-up input. `steer()` puts input in the next-step inbox and wakes the driver. During a turn, the loop can claim it at a later step boundary; while idle, the waking next-step batch starts a new turn. Input arriving after a batch was claimed waits for a later boundary, while cancellation or disposal can discard it.
 
@@ -35,7 +35,7 @@ The no-batching rule applies only to ordinary follow-up input. `steer()` puts in
 - Unit and property tests submit sends from the same stack, neighboring microtasks, different producers, and reentrant callbacks; every message gets its own FIFO-ordered turn.
 - A built-stdio test submits two lines and observes two model requests and two turn boundaries.
 - Delayed and rejected first-turn checkpoints keep the next turn waiting and prove that its request sees the preceding assistant result.
-- Failure-path tests cover pre-step rejection, listener failure, broad cancellation, disposal, and failure before `turn/start`; rejection creates no turn, recorded turns stay balanced, messages do not merge, and surviving later work still drains.
+- Failure-path tests cover pre-step rejection, listener failure, broad cancellation, disposal, and failure before `turn/start`; initial pre-step exits close balanced no-step turns, messages do not merge, and surviving later work still drains.
 - Separate tests cover open-turn, failed-turn, and idle `steer()`, pending `inject()`, whole-agent status, and `whenIdle()`.
 
 ## Consequences

.agents/notes/implemented/simplification/2026-07-17-one-send-one-turn.zh.md

@@ -20,7 +20,7 @@ Status: implemented
 
 如果消息 A、B 都进入处理，B 的轮次只能在 A 记录 `turn/end` 且 A 的持久性检查点处理结束后开始。因此，B 的请求能看到 A 在同一会话日志中留下的已关闭结果。检查点错误会照常报告，但处理结束只表示解除这道顺序屏障，不表示失败的写入已经持久化。广义 `cancel()`、dispose（资源释放）或 `turn/start` 之前的失败也可能丢弃尚未启动的队列项，而不打开一个空轮次。
 
-轮次边界上，循环会在待处理 next-step 输入之后领取一条 follow-up。`agent/pre-step` 要么拒绝提案，要么返回进入步骤的完整批次。被拒绝的 follow-up 保持已删除，不打开轮次，也不写入会话历史。实现中不存在混合普通 follow-up 分支。
+轮次边界上，循环会先打开轮次，再在待处理 next-step 输入之后领取一条 follow-up。`agent/pre-step` 要么拒绝提案，要么返回进入步骤的完整批次。被拒绝的 follow-up 保持已删除，并关闭一个 blocked 的无步骤轮次，不写入模型可见历史。实现中不存在混合普通 follow-up 分支。
 
 上述不合批规则只适用于普通 follow-up 输入。`steer()` 会把输入放入 next-step inbox 并唤醒驱动器。在轮次期间，循环可以在后续步骤边界领取它；agent 空闲时，这个会唤醒的 next-step 批次会启动一个新轮次。批次被领取后才到达的输入会等待后续边界，而取消或 dispose 可以将其丢弃。
 
@@ -35,7 +35,7 @@ Status: implemented
 - 单元测试和性质测试从同一调用栈、相邻微任务、不同生产方和重入回调提交 send；每条消息都会得到一个按 FIFO 排序的独立轮次。
 - stdio 构建产物测试提交两行输入，并观察到两个模型请求和两个轮次边界。
 - 延迟和拒绝第一个轮次的检查点，都能让下一个轮次保持等待，并证明其请求可以看到前一条助手结果。
-- 失败路径测试覆盖 pre-step reject、监听器失败、广义取消、dispose 和 `turn/start` 之前的失败；reject 不会创建轮次，已记录的轮次保持边界平衡，消息不会合并，之后仍需处理的工作也能继续清空。
+- 失败路径测试覆盖 pre-step reject、监听器失败、广义取消、dispose 和 `turn/start` 之前的失败；首次 pre-step 的各种退出都会关闭边界平衡的无步骤轮次，消息不会合并，之后仍需处理的工作也能继续清空。
 - 其他测试分别覆盖轮次打开时、轮次失败后和空闲时的 `steer()`，以及待处理的 `inject()`、面向整个 agent 的状态和 `whenIdle()`。
 
 ## 后果