Add plugin isolation IPC scaffolding and host phase one docs (#5)

LanMountainDesktop.PluginSdk/PluginRuntimeModes.cs

@@ -0,0 +1,58 @@
+namespace LanMountainDesktop.PluginSdk;
+
+public static class PluginRuntimeModes
+{
+    public const string InProcess = "in-proc";
+    public const string IsolatedBackground = "isolated-background";
+    public const string IsolatedWindow = "isolated-window";
+
+    public static bool TryParse(string? value, out PluginRuntimeMode mode)
+    {
+        switch (value?.Trim().ToLowerInvariant())
+        {
+            case null:
+            case "":
+            case InProcess:
+                mode = PluginRuntimeMode.InProcess;
+                return true;
+            case IsolatedBackground:
+                mode = PluginRuntimeMode.IsolatedBackground;
+                return true;
+            case IsolatedWindow:
+                mode = PluginRuntimeMode.IsolatedWindow;
+                return true;
+            default:
+                mode = default;
+                return false;
+        }
+    }
+
+    public static PluginRuntimeMode Parse(string? value, string sourceName, string propertyName = "runtime.mode")
+    {
+        if (TryParse(value, out var mode))
+        {
+            return mode;
+        }
+
+        var candidate = string.IsNullOrWhiteSpace(value) ? "<empty>" : value.Trim();
+        throw new InvalidOperationException(
+            $"Plugin manifest '{sourceName}' declares unsupported runtime mode '{candidate}' in '{propertyName}'. " +
+            $"Supported values: '{InProcess}', '{IsolatedBackground}', '{IsolatedWindow}'.");
+    }
+
+    public static string NormalizeManifestValue(string? value, string sourceName, string propertyName = "runtime.mode")
+    {
+        return ToManifestValue(Parse(value, sourceName, propertyName));
+    }
+
+    public static string ToManifestValue(PluginRuntimeMode mode)
+    {
+        return mode switch
+        {
+            PluginRuntimeMode.InProcess => InProcess,
+            PluginRuntimeMode.IsolatedBackground => IsolatedBackground,
+            PluginRuntimeMode.IsolatedWindow => IsolatedWindow,
+            _ => throw new ArgumentOutOfRangeException(nameof(mode), mode, "Unsupported plugin runtime mode.")
+        };
+    }
+}