feat(m4): single instance IPC

- LaunchCommand: ShowMain / OpenWorkspace(path), doubles as IPC message
- LaunchArguments: first non-blank arg wins, blank-only args -> ShowMain
- IpcProtocol: strict newline-line codec (show / open+path), tolerant
  of trailing-newline framing only
- SingleInstanceService: per-session named mutex decides primary;
  primary serves a named-pipe loop (decode -> dispatch -> ack) with
  handler failures contained; secondary retries send, exits on ack
- Program/App wiring: secondary forwards command and exits; primary
  activates the main window for forwarded commands (workspace switch
  deferred to the workspace UI milestone)
- 15 new tests (protocol codec, argument parsing); smoke-verified:
  instance 1 stays alive, instance 2 self-exits with code 0

YKanBan.Tests/Ipc/LaunchArgumentsTests.cs

@@ -0,0 +1,47 @@
+using YKanBan.Ipc;
+
+namespace YKanBan.Tests.Ipc;
+
+/// <summary>Tests for command-line argument parsing.</summary>
+[TestClass]
+public class LaunchArgumentsTests
+{
+    [TestMethod]
+    public void NoArgumentsYieldShowMain()
+    {
+        Assert.AreEqual(new LaunchCommand.ShowMain(), LaunchArguments.Parse([]));
+    }
+
+    [TestMethod]
+    public void BlankArgumentsOnlyYieldShowMain()
+    {
+        Assert.AreEqual(new LaunchCommand.ShowMain(), LaunchArguments.Parse(["", "  "]));
+    }
+
+    [TestMethod]
+    public void SingleArgumentBecomesWorkspacePath()
+    {
+        Assert.AreEqual(
+            new LaunchCommand.OpenWorkspace(@"D:\repo"),
+            LaunchArguments.Parse([@"D:\repo"]));
+    }
+
+    [TestMethod]
+    public void PathWithSpacesIsPreserved()
+    {
+        Assert.AreEqual(
+            new LaunchCommand.OpenWorkspace(@"D:\my repo\ws"),
+            LaunchArguments.Parse([@"D:\my repo\ws"]));
+    }
+
+    [TestMethod]
+    public void FirstMeaningfulArgumentWins()
+    {
+        Assert.AreEqual(
+            new LaunchCommand.OpenWorkspace(@"D:\a"),
+            LaunchArguments.Parse([@"D:\a", @"D:\b"]));
+        Assert.AreEqual(
+            new LaunchCommand.OpenWorkspace(@"D:\b"),
+            LaunchArguments.Parse(["", @"D:\b", @"D:\c"]));
+    }
+}