Desktop: complete Runtime Host message queue parity

[M4n5ter]

English

Context

#2222 adds mid-turn steering for the embedded Desktop runtime. It solves the current-turn path, including same-turn continuation and live transcript presentation.

The work only partially overlaps with the remaining Desktop queue slice. In particular, #2222 does not connect Host-backed Desktop sessions to the Runtime Host message authority, and it intentionally removed its earlier component-local queue UI.

Remaining gap

Host-backed Desktop needs to use the Runtime Host as the single authority for message admission and queue state:

• submit ordinary text through turn.message.submit so the Host atomically decides between current-turn steering and starting a successor turn;
• expose an explicit next-turn queue while a response is active;
• mirror authoritative queue_update state per Session, including reconnect hydration;
• allow queued text to be retracted back into the composer;
• restore retractable queued text when Stop interrupts a turn;
• preserve equivalent automatic follow-up behavior for the embedded runtime;
• keep behavior correct when multiple clients observe or submit to the same Session.

This is complementary to #2222: the current-turn embedded behavior should land there, while this slice supplies the shared Desktop controls and Runtime Host integration.

Implementation status

A validated implementation is available on feat/desktop-message-queue.

The branch currently includes #2222 as its implementation base. After #2222 merges, it should be rebased onto main and published as an independent PR rather than as a cross-fork stack.

Acceptance criteria

• Embedded and Host-backed Desktop expose the same current-turn and next-turn message behavior.
• Host-backed sends do not race a client-side active-turn snapshot.
• Queue state remains authoritative across multiple connected clients.
• Retract and Stop do not silently discard queued text.
• Normal SessionEvent ordering is unchanged when no between-turn queue mutation occurred.
• Runtime, Desktop, and UI regression suites pass.

中文

背景

#2222 为 embedded Desktop runtime 增加了运行中 steering，解决了当前轮路径，包括同轮继续执行和实时对话记录展示。

这项工作与剩余的 Desktop 消息队列 slice 只有部分重叠。具体而言，#2222 没有把 Host-backed Desktop 会话接入 Runtime Host 的消息权威，也按评审意见移除了此前位于组件内部的本地队列 UI。

尚存缺口

Host-backed Desktop 需要以 Runtime Host 作为消息准入和队列状态的唯一权威：

• 普通文本通过 turn.message.submit 提交，由 Host 原子决定注入当前轮还是启动后继轮；
• 回答进行期间提供明确的下一轮消息队列；
• 按 Session 镜像权威 queue_update 状态，并支持重连后的状态恢复；
• 允许把排队文本撤回输入框；
• Stop 中断当前轮时，将仍可撤回的排队文本恢复到输入框；
• 为 embedded runtime 保留等价的自动 follow-up 行为；
• 多个客户端观察或向同一 Session 提交消息时仍保持正确行为。

本项工作是 #2222 的补充：当前轮 embedded 行为应由 #2222 合入，本 slice 则补齐共享 Desktop 控件和 Runtime Host 集成。

实现状态

已验证的实现位于 feat/desktop-message-queue 分支。

该分支目前以 #2222 的实现为基础。#2222 合并后，应将该分支 rebase 到 main，再作为独立 PR 发布，而不是创建 cross-fork stack。

验收标准

• Embedded 与 Host-backed Desktop 提供一致的当前轮和下一轮消息行为。
• Host-backed 发送不会与客户端侧的活动轮快照发生竞态。
• 多客户端连接时，队列状态仍由权威来源统一维护。
• Retract 和 Stop 不会静默丢弃排队文本。
• 没有跨轮队列变化时，正常 SessionEvent 顺序保持不变。
• Runtime、Desktop 和 UI 回归测试全部通过。

[Astro-Han]

Closing as completed. The Runtime Host is now the sole message and queue authority: ordinary submissions use turn.message.submit, authoritative queue_update state drives Desktop, busy-send races are handled at the Host boundary, and #3513/#3544 added the user-facing queue plus per-entry promote, retract, restore-to-draft, and reorder operations.

The original embedded-runtime parity clause predates the Runtime Host production cutover and no longer represents a separate supported execution authority.

Triage and wording were prepared with OpenAI Codex and reviewed by the maintainer.