fix: preserve layout loading state across temporary overlays

[google-labs-jules]

观察到的问题

在 Daily Lifecycle Review 中发现：ChatConversationSurfaceProjectionCoordinator 没有将界面可视状态（ShouldShow...）与界面布局加载状态（ShouldLoad...，在 Uno/WinUI 中对应 x:Load）解耦。临时显示阻塞遮罩或切换会话时，底层会话根和消息记录的 ShouldShow 会变为 false，导致同名的 ShouldLoad 跟着变 false，从而在 UI 层卸载整个结构，后续又需要重新挂载，违背了框架的最佳实践和加载状态的单调性（monotonicity）。

根本原因

ChatConversationSurfaceProjectionCoordinator 直接使用了展示状态作为加载状态，没有通过内部状态跟踪来记录“是否曾经挂载过”。只要曾经需要显示，就应该保持挂载状态。

为什么架构上是安全的

我引入了两个布尔类型的本地字段 _hasLoadedActiveConversationRoot 和 _hasLoadedTranscriptSurface，它们只会被设为 true。通过将这两个字段传给 ChatConversationSurfaceProjection 作为加载依据，保证了单调性。这种改动保持了单一事实源（SSOT）设计，也没有在 Code-Behind 中加入任何黑客代码，并且符合跨平台的 x:Load 生命周期控制最佳实践。

验证命令

我执行了以下的验证步骤：

1. 编译解决方案：dotnet build SalmonEgg.sln --configuration Debug（发现缺少的 WebAssembly workload 后，执行了 dotnet workload install wasm-tools 修复编译环境）。
2. 验证修复是否通过原测试逻辑（修改了有缺陷的断言来保证新状态）：dotnet test tests/SalmonEgg.Presentation.Core.Tests/SalmonEgg.Presentation.Core.Tests.csproj --configuration Debug --no-restore
3. 执行全局测试验证契约：dotnet test SalmonEgg.sln --configuration Debug --no-build

剩余未验证的仅限 GUI 的风险

依赖了底层依赖的 Uno 平台对于 x:Load 在 ShouldLoad 保持 true 并且 Visibility 设为 Collapsed 时的开销，这需要在不同平台的实际渲染管线上做 smoke testing 才能最后确认其完全零回归。然而架构上的变化是纯正的逻辑变更，不引入任何特定于平台的变化。

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PR created automatically by Jules for task 14034288579553654798 started by @YoungSx

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Project	Deployment	Actions	Updated (UTC)
salmonegg	Ready	Preview, Comment	Jun 21, 2026 12:20am

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[YoungSx]

Closing during PR triage: this is a stale or duplicate layout-loading monotonicity attempt. #49 is the clean green candidate for this behavior; this PR is superseded, failing/conflicting, or mixed-scope and should not be merged as-is.

[google-labs-jules]

Closing during PR triage: this is a stale or duplicate layout-loading monotonicity attempt. #49 is the clean green candidate for this behavior; this PR is superseded, failing/conflicting, or mixed-scope and should not be merged as-is.

Understood. Acknowledging that this work is now obsolete and stopping work on this task.