fix: stop backend write thread before teardown in ~Settings

[add-uos]

问题

CI UT 任务中 test_settings 套件段错误（exit 139，SIGSEGV），gtest XML 报告未写出，套件内 66 个用例整体丢失：

[ RUN      ] SettingsTest.BUG102351_SettingsDestroyed_ConfigValuePreserved
Destroying Settings instance
Cleaning up settings backend
QMutex: destroying locked mutex        ← 未定义行为警告
Settings instance destroyed
CustemBackend::CustemBackend "..."     ← 测试新建验证后端读盘
Entering CustemBackend::keys
段错误（核心转储）

根因

• DSettings::setBackend() 会将 backend moveToThread 到专门的写线程，doSetOption/doSync 经队列在该线程异步执行（dtkcore 源码确认）；
• 只有 DSettings::destroyed 信号会 quit()+wait() 该写线程；
• 原 ~Settings() 只 delete m_backend（backend 的线程亲和在写线程上），且成员 settings（DSettings*）从不析构 → 写线程永不停止，主线程销毁 backend 及其 QSettings 时与写线程在途的 doSetOption 竞态。

core dump 堆栈（崩溃线程为 DTK 写线程，非主线程）：

#0  QSettings::setValue(...)                     ← UAF
#1  Dtk::Core::QSettingBackend::doSetOption(...)
#2  QObject::event(QEvent*)
#12 QThread::exec()                              ← 写线程事件循环

修复

~Settings() 中先 delete settings（触发 DSettings::destroyed → 写线程 quit+wait 停稳），再 delete m_backend；同时修复 DSettings 实例泄漏。

验证

复刻 CI 运行环境（cwd=build、ASAN_OPTIONS、--gtest_output=xml、全量 66 用例）：

	修复前	修复后
全量运行	2/2 必现 exit 139	5/5 全部 exit 0
QMutex: destroying locked mutex	每次出现	0 次

单跑 BUG102351 用例不触发（竞态依赖前序用例留下的在途写事件），需全量套件复现。

Summary by Sourcery

Stop the settings backend write thread before tearing down the backend to make destruction safe.

Bug Fixes:

• Prevent settings teardown from racing with asynchronous backend writes, eliminating the CI segmentation fault and locked-mutex warning.

Enhancements:

• Release the DSettings instance during destruction and ensure its write thread is stopped before the backend is deleted.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

修复 Settings 析构期间 backend 写线程仍在异步访问已销毁 QSettings 的竞态：析构时先释放 DSettings 以停止并等待写线程，再释放 backend，并消除 DSettings 泄漏；全量 test_settings 验证由稳定崩溃恢复为连续通过。

Sequence diagram for safe Settings teardown

sequenceDiagram
    participant Settings
    participant DSettings
    participant WriteThread
    participant Backend
    participant QSettings

    Settings->>DSettings: delete settings
    DSettings->>WriteThread: destroyed: quit()
    DSettings->>WriteThread: wait()
    WriteThread-->>DSettings: stopped
    Settings->>Backend: delete m_backend
    Backend->>QSettings: destroy safely

Loading

File-Level Changes

Change	Details	Files
调整 Settings 析构顺序，先停止并等待 DSettings 管理的 backend 写线程，再释放 backend，避免异步写入与后端销毁并发导致 UAF 和崩溃。	在析构函数中删除 settings，触发 DSettings::destroyed 对写线程执行 quit+wait。 将 settings 指针置空后再删除 m_backend，确保后端及其 QSettings 不再被后台线程访问。 同时修复 settings 裸指针未释放造成的 DSettings 实例泄漏。	src/common/settings.cpp

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[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: add-uos, lzwind

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