feat(D.2b): RuntimeOptions.RetailUi + AcDir toggles

Adds two startup-time env toggles that Phase D.2b's retail-UI panel
frame will read:
- ACDREAM_RETAIL_UI=1  → opts.RetailUi (bool, default false)
- ACDREAM_AC_DIR=<path> → opts.AcDir   (string?, default null)

Both follow the existing helper conventions (IsExactlyOne / NullIfEmpty).
No call sites broke because the only construction site in RuntimeOptions.cs
already uses named arguments.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/AcDream.App.Tests/RuntimeOptionsRetailUiTests.cs

@@ -0,0 +1,28 @@
+using System.Collections.Generic;
+using AcDream.App;
+
+namespace AcDream.App.Tests;
+
+public class RuntimeOptionsRetailUiTests
+{
+    [Fact]
+    public void Parse_ReadsRetailUiAndAcDir()
+    {
+        var env = new Dictionary<string, string?>
+        {
+            ["ACDREAM_RETAIL_UI"] = "1",
+            ["ACDREAM_AC_DIR"] = @"C:\Turbine\Asheron's Call",
+        };
+        var opts = RuntimeOptions.Parse("dats", k => env.GetValueOrDefault(k));
+        Assert.True(opts.RetailUi);
+        Assert.Equal(@"C:\Turbine\Asheron's Call", opts.AcDir);
+    }
+
+    [Fact]
+    public void Parse_DefaultsRetailUiOffAndAcDirNull()
+    {
+        var opts = RuntimeOptions.Parse("dats", _ => null);
+        Assert.False(opts.RetailUi);
+        Assert.Null(opts.AcDir);
+    }
+}