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

LanMountainDesktop.PluginIsolation.Contracts/DiagnosticsContracts.cs

@@ -0,0 +1,45 @@
+namespace LanMountainDesktop.PluginIsolation.Contracts;
+
+public sealed record PluginHeartbeatPing(
+    string SessionId,
+    DateTimeOffset SentAtUtc);
+
+public sealed record PluginHeartbeatPong(
+    string SessionId,
+    DateTimeOffset ReceivedAtUtc);
+
+public sealed record PluginLogEntry(
+    string Level,
+    string Category,
+    string Message,
+    DateTimeOffset TimestampUtc,
+    string? Exception = null);
+
+public static class PluginLogLevels
+{
+    public const string Trace = "trace";
+    public const string Debug = "debug";
+    public const string Information = "information";
+    public const string Warning = "warning";
+    public const string Error = "error";
+    public const string Critical = "critical";
+}
+
+public sealed record PluginFaultReport(
+    string SessionId,
+    string FaultKind,
+    bool IsFatal,
+    string Message,
+    string? StackTrace = null,
+    int? WorkerProcessId = null,
+    int? ExitCode = null,
+    DateTimeOffset? OccurredAtUtc = null);
+
+public static class PluginFaultKinds
+{
+    public const string ManagedException = "managed-exception";
+    public const string NativeCrash = "native-crash";
+    public const string WatchdogTimeout = "watchdog-timeout";
+    public const string StartupFailure = "startup-failure";
+    public const string ForcedTermination = "forced-termination";
+}