feat(cpp): sans-IO round engine for the native cold-start round

[sunnylqm]

RoundEngine 系列的第 1 步（共享原生轮次引擎）：只加 C++ 引擎、轨迹向量与设计说明，不改任何平台代码。

为什么

三端编排器（Android SyncCoordinator.java、iOS RCTPushyOrchestrator.mm、鸿蒙 SyncCoordinator.ts）各自手写了串联共享步骤的驱动循环。向量只锁住了被调用的纯函数，锁不住"怎么调用"，2026-10 架构审查发现的几处漂移（错误码分类、连接超时、prepareBundle 超时、激活前摘要门）都出在这一层。

改了什么

• cpp/update_flow_core/round_engine.{h,cpp}：StepRound(state, event) -> {state, commands}。
  • 纯函数，不做 IO、不读时钟、不加锁。时间随事件传入（now，单调毫秒）。
  • 状态是 JSON 值，JNI/NAPI 绑定只传文本，不持有句柄；鸿蒙每步都是一次同步的毫秒级调用。
  • 引擎负责：配置门、面包屑时机、query 遍历与单请求超时（空闲 10s / 整次 15s，按预算钳制）、hash 路径校验、已安装短路、双下载预算、镜像重试（PATCH_FAILED 换策略）、激活判定与 crash-hold 标记、结果映射。
  • 另有宿主等待后的取消判定 HostCancellation（鸿蒙原先用字符串全等，统一为 C1 等价）。
• tests/round-engine-vectors.json：32 条事件轨迹（无更新、端点回退、远程列表、三种激活来源、镜像重试、无码失败、全部失败、两种预算耗尽、full 超时、reset 取消、提交失败、无运行版本、不安全 hash、crash hold 带预算、轮中开始 hold、预算在 query/下载前耗尽、forceActivate 被丢弃、协议错误）+ 8 条宿主取消 + 10 条路径校验。
• C++ 测试经 Value API 与 JSON 绑定两条路径回放每一步，并断言每条轨迹都走到 done；fuzz 目标增加了"用模糊响应驱动整轮"。
• docs/NATIVE_CHECKUPDATE_DESIGN.md 新增 §12：契约（事件/命令表）、引擎与平台的分工、相对现有手写循环的有意对齐，以及分期计划。

验证

• SANITIZE=1 ./scripts/test-update-flow-core.sh 全部通过（ASan + UBSan）。故意篡改一条向量，测试会报 MISMATCH。
• FUZZ=1 跑了 50s，没有发现问题。
• 按三端 release 参数编译 round_engine.cpp（Android -std=c++17 -fexceptions -fno-rtti -DNDEBUG、iOS -std=c++20 -DNDEBUG、-fno-exceptions）均干净；check-binary-strings 检查产物无敏感字样。
• bun run lint、bun test（438 个）均通过。

后续

• PR 2：补 CODE_AUDIT 4.4，即鸿蒙 NAPI 与 iOS .mm 入口的 C++ catch (...)。
• PR 3～5：三端依次接入，顺序为 Android → iOS → 鸿蒙。Android 和鸿蒙要等 feat(native): bounded prepareBundle on Android and HarmonyOS #669 合入后再动。

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Summary by CodeRabbit

• Improvements
  • Update checks and downloads now handle validation, retries across available sources, timeouts, and configuration changes consistently.
  • Update outcomes account for installed versions, activation settings, and commit results.
• Documentation
  • Added an overview of update-flow behavior and the responsibilities handled by platform-specific operations.

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📒 Files selected for processing: 5

• cpp/update_flow_core/native_round.cpp
• cpp/update_flow_core/round_engine.cpp
• cpp/update_flow_core/round_engine.h
• cpp/update_flow_core/tests/round-engine-vectors.json
• docs/NATIVE_CHECKUPDATE_DESIGN.md

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📝 Walkthrough

The pull request adds a shared C++ round engine that returns state updates and commands from events. It adds test vectors, test and fuzz coverage, and design documentation. Platform orchestrators are not converted in this change.

Changes: Native Update Round Engine

Layer / File(s)	Summary
Engine contract and round entry cpp/update_flow_core/round_engine.h, cpp/update_flow_core/round_engine.cpp, cpp/update_flow_core/native_round.h, docs/NATIVE_CHECKUPDATE_DESIGN.md	The interface defines round state, events, commands, and adapter responsibilities. The engine initializes rounds, prepares environment data, and emits endpoint-query commands. The design document describes the division of work between the engine and adapters. The native_round.h change updates a comment about native responsibilities.
Response, download, and commit processing cpp/update_flow_core/round_engine.cpp, cpp/update_flow_core/native_round.cpp, docs/NATIVE_CHECKUPDATE_DESIGN.md	The engine validates responses, checks installed versions, advances download strategies and mirrors, and emits commit commands. It maps commit results, validates paths and JSON inputs, and checks host cancellation. Counter now caps valid values at 1e9. The design document lists behavior alignments with prior platform loops.
Engine vectors and test coverage cpp/update_flow_core/tests/*, scripts/test-update-flow-core.sh, docs/NATIVE_CHECKUPDATE_DESIGN.md	The vector suite and C++ tests cover event traces, cancellation, path validation, and malformed state. The fuzz harness drives the engine. The test script loads the new vectors, and the design document records a staged rollout.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~45 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant PlatformAdapter
  participant StepRound
  participant PlatformIO
  PlatformAdapter->>StepRound: Send start or outcome event
  StepRound-->>PlatformAdapter: Return state and commands
  PlatformAdapter->>PlatformIO: Execute command
  PlatformIO-->>PlatformAdapter: Return operation outcome
  PlatformIO->>PlatformAdapter: Provide operation outcome
  PlatformAdapter->>StepRound: Send outcome event

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Merge Risk: ⚪ Minimal · up to 05c8a

The previously identified cancellation-result mismatch is corrected. No remaining merge-blocking issue is established.

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Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 35.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 40 functions across 7 files. (1 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: adding a sans-IO C++ round engine for the native cold-start round.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 35.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 40 functions across 7 files. (1 skipped: 1 unsupported.)

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Actionable comments posted: 3

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• 🪄 Fix comments: Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
Review comments at @cpp/update_flow_core/round_engine.cpp:
- Around line 377-378: Update the `committed` event handling so a failed
config-generation check is identified separately from a reset, and map that
reason to `config_changed` when finishing the round; preserve `reset` for reset
failures. Add the requested vector for the config-changed case, using the
existing vector pattern.
- Around line 96-98: Update the event dispatch branch in RoundEngine so
OnCommitted() runs only when the event type is the expected committed phase. For
any other unmatched phase, including an empty type, finish the round as failed
with internal_error.
- Around line 46-47: Update Index to clamp numeric inputs to the representable
size_t range before conversion, avoiding out-of-range floating-point casts. In
the timeout path that sets the attempt cursor, use the attempts collection’s
Size() as the past-the-end cursor instead of the 1e15 sentinel.

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• cpp/update_flow_core/native_round.h
• cpp/update_flow_core/round_engine.cpp
• cpp/update_flow_core/round_engine.h
• cpp/update_flow_core/tests/fuzz_flow_json.cpp
• cpp/update_flow_core/tests/round-engine-vectors.json
• cpp/update_flow_core/tests/update_flow_core_test.cpp
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• scripts/test-update-flow-core.sh

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