Fix: Source generator respects RootNamespace MSBuild property

[Copilot]

The source generator was ignoring the RootNamespace property in .csproj files, always using the assembly name for the generated R class namespace instead.

Changes

MSBuild Integration

• Added RootNamespace to CompilerVisibleProperty in LuYao.ResourcePacker.MSBuild.props to expose the property to source generators

Source Generator

• Modified GetRootNamespace() to read build_property.RootNamespace from AnalyzerConfigOptionsProvider
• Falls back to assembly name when RootNamespace is not set

Verification

• Added samples/RootNamespaceTest demonstrating the fix with RootNamespace=Popcorn.Toolkit

Example

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <RootNamespace>Popcorn.Toolkit</RootNamespace>
    <ResourcePackerEnabled>true</ResourcePackerEnabled>
  </PropertyGroup>
</Project>

Generated code now correctly uses the specified namespace:

namespace Popcorn.Toolkit  // Previously would be assembly name
{
    internal static class R
    {
        public static class Keys { ... }
        public static ResourcePackageReader Reader { get; }
    }
}

Original prompt

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This section details on the original issue you should resolve

<issue_title>源生成器没有正确识别 RootNamespace 属性</issue_title>
<issue_description>当在 .csproj 文件中设定 RootNamespace 属性后，SG 项目生成的 R 类型所在命名空间没有遵循该属性，仍为程序集名称。

比如：

<Project Sdk="Microsoft.NET.Sdk">

	<PropertyGroup>
		<TargetFramework>net8.0</TargetFramework>
		<ImplicitUsings>enable</ImplicitUsings>
		<Nullable>enable</Nullable>
		<RootNamespace>Popcorn.Toolkit</RootNamespace>
	</PropertyGroup>

</Project>

生成的代码：

// <auto-generated />
#nullable enable

using System;
using System.IO;
using LuYao.ResourcePacker;

namespace Popcorn.Toolkit.Client
{
    /// <summary>
    /// Provides strongly-typed access to resources in Popcorn.Toolkit.Client.
    /// </summary>
    internal static class R

请创建一个项目用来验证、测试。项目存放在 samples 目录，并加入解决方案。</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes 源生成器没有正确识别 RootNamespace 属性 #31

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