Add plugin isolation IPC scaffolding and host phase one docs

LanMountainDesktop.Tests/PluginManifestRuntimeTests.cs

@@ -0,0 +1,48 @@
+using System.Text;
+using LanMountainDesktop.PluginSdk;
+using Xunit;
+
+namespace LanMountainDesktop.Tests;
+
+public sealed class PluginManifestRuntimeTests
+{
+    [Fact]
+    public void Load_WhenRuntimeIsMissing_DefaultsToInProcess()
+    {
+        const string json = """
+            {
+              "id": "plugin.runtime.default",
+              "name": "Runtime Default",
+              "entranceAssembly": "Plugin.dll"
+            }
+            """;
+
+        using var stream = new MemoryStream(Encoding.UTF8.GetBytes(json));
+        var manifest = PluginManifest.Load(stream, "plugin.json");
+
+        Assert.NotNull(manifest.Runtime);
+        Assert.Equal(PluginRuntimeModes.InProcess, manifest.Runtime!.Mode);
+        Assert.Equal(PluginRuntimeMode.InProcess, manifest.RuntimeMode);
+    }
+
+    [Fact]
+    public void Load_WhenRuntimeModeIsInvalid_ThrowsHelpfulError()
+    {
+        const string json = """
+            {
+              "id": "plugin.runtime.invalid",
+              "name": "Runtime Invalid",
+              "entranceAssembly": "Plugin.dll",
+              "runtime": {
+                "mode": "shared-worker"
+              }
+            }
+            """;
+
+        using var stream = new MemoryStream(Encoding.UTF8.GetBytes(json));
+        var ex = Assert.Throws<InvalidOperationException>(() => PluginManifest.Load(stream, "plugin.json"));
+
+        Assert.Contains("runtime.mode", ex.Message);
+        Assert.Contains("shared-worker", ex.Message);
+    }
+}