feat(D.2b): run importer-built vitals window under ACDREAM_RETAIL_UI_IMPORTER (A/B)

Adds RuntimeOptions.RetailUiImporter (ACDREAM_RETAIL_UI_IMPORTER=1) — a new
opt-in flag that runs the LayoutImporter-built vitals window ALONGSIDE the
hand-authored vitals panel for pixel-for-pixel A/B comparison. The importer
window is placed at x=200, y=30 so both render simultaneously within the same
ACDREAM_RETAIL_UI=1 session. The hand-authored path is entirely untouched and
remains the default; the importer path is the eventual switch-over target.

Also adds two RuntimeOptionsRetailUiTests covering the new flag: value "1" →
true, unset/other → false.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/AcDream.App/RuntimeOptions.cs

@@ -41,6 +41,7 @@ public sealed record RuntimeOptions(
     bool DumpLiveSpawns,
     int? LegacyStreamRadius,
     bool RetailUi,
+    bool RetailUiImporter,
     string? AcDir)
 {
     /// <summary>
@@ -85,6 +86,7 @@ public sealed record RuntimeOptions(
             // top of the quality preset's radii. Null when unset or invalid.
             LegacyStreamRadius:  TryParseNonNegativeInt(env("ACDREAM_STREAM_RADIUS")),
             RetailUi:            IsExactlyOne(env("ACDREAM_RETAIL_UI")),
+            RetailUiImporter:    IsExactlyOne(env("ACDREAM_RETAIL_UI_IMPORTER")),
             AcDir:               NullIfEmpty(env("ACDREAM_AC_DIR")));
     }