feat: unified sample hub — consolidate all samples into a single interactive CLI

Consolidate all 9 individual sample projects into a single Hub console
application with an interactive menu. Each sample demonstrates a different
GeneralUpdate component API end-to-end.

**New:**
- src/Hub/ — unified sample launcher with interactive menu
- src/Hub/Samples/ — all 9 samples as ISample implementations
- src/Hub/AppConfig.cs — JSON configuration model
- src/Hub/appsettings.json — server & product configuration
- src/Run.cmd / src/Run.ps1 — one-click launcher scripts
- src/gen_packages.ps1 — test package generator
- src/content_client/ — test data for Differential sample
- src/content_upgrade/ — test data for Upgrade packages
- src/content/ — intermediate version test data
- Directory.Build.props — shared build properties
- global.json — .NET 10 SDK pinning

**Samples & their GeneralUpdate API usage:**
1. CompleteUpdate — GeneralUpdateBootstrap (AppType.Client)
2. SilentUpdate — GeneralUpdateBootstrap (AppType.Client, silent)
3. OSS — GeneralUpdateBootstrap (AppType.OssClient)
4. Differential — DiffPipelineBuilder + BsdiffDiffer
5. Push — UpgradeHubService (SignalR push client)
6. Bowl — BowlContext (process monitor/dump)
7. Extension — GeneralExtensionHost (plugin system)
8. Drivelution — GeneralDrivelution (driver management)
9. Compress — CompressProvider (zip compress/decompress)

**Removed:** old individual sample projects (Bowl, Client, Compress,
Diff, Drivelution, Extension, OSS, Push, Upgrade) — all consolidated
under src/Hub/.

**Fixed:**
- Removed unnecessary PackageReferences already provided by ASP.NET Core
- Added OperatingSystem.IsWindows() guard in DrivelutionSample (CA1416)
- Cleaned up unused GuLibsPath property in Directory.Build.props

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: JusterZhu

Directory.Build.props

@@ -0,0 +1,8 @@
+<Project>
+  <PropertyGroup>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <LangVersion>latest</LangVersion>
+    <ManagePackageVersionsCentrally>false</ManagePackageVersionsCentrally>
+  </PropertyGroup>
+</Project>

cmd.txt

@@ -0,0 +1,7 @@
+{
+  "sdk": {
+    "version": "10.0.201",
+    "rollForward": "latestMajor",
+    "allowPrerelease": true
+  }
+}