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>

src/AcDream.App/RuntimeOptions.cs

@@ -39,7 +39,9 @@ public sealed record RuntimeOptions(
     bool RetailCloseDegrades,
     bool DumpSceneryZ,
     bool DumpLiveSpawns,
-    int? LegacyStreamRadius)
+    int? LegacyStreamRadius,
+    bool RetailUi,
+    string? AcDir)
 {
     /// <summary>
     /// Build options from the process environment. Used by
@@ -81,7 +83,9 @@ public sealed record RuntimeOptions(
             DumpLiveSpawns:      IsExactlyOne(env("ACDREAM_DUMP_LIVE_SPAWNS")),
             // Legacy override for ACDREAM_STREAM_RADIUS. Caller applies it on
             // top of the quality preset's radii. Null when unset or invalid.
-            LegacyStreamRadius:  TryParseNonNegativeInt(env("ACDREAM_STREAM_RADIUS")));
+            LegacyStreamRadius:  TryParseNonNegativeInt(env("ACDREAM_STREAM_RADIUS")),
+            RetailUi:            IsExactlyOne(env("ACDREAM_RETAIL_UI")),
+            AcDir:               NullIfEmpty(env("ACDREAM_AC_DIR")));
     }
 
     /// <summary>True iff live-mode credentials are present and valid for connecting.</summary>