test(D.2b): conformance polish — table-driven slice asserts + BOM-safe loader

Fix 1: replace 3 copy-paste meter blocks in VitalsTree_MetersHaveExpectedSliceIds
with a single table-driven loop — a 4th meter is now a one-liner and failures
name the failing meter id directly.

Fix 2: FixtureLoader now reads the fixture as bytes and strips the UTF-8 BOM
(EF BB BF) before passing the span to JsonSerializer, so a BOM-bearing fixture
file never causes a spurious JsonReaderException.

Fix 3: add [Trait("Category", "Conformance")] at the class level so conformance
tests are selectable by category filter.

Fix 4: add missing <param name="layoutId"> doc tag to LayoutImporter.ImportInfos.

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

src/AcDream.App/UI/Layout/LayoutImporter.cs

@@ -129,6 +129,8 @@ public static class LayoutImporter
     /// ElementInfo tree (no widgets). Exposed for fixture generation + conformance tests.
     /// Returns null if the layout is missing.
     /// </summary>
+    /// <param name="dats">The dat collection to read the LayoutDesc from.</param>
+    /// <param name="layoutId">The LayoutDesc dat id to read.</param>
     public static ElementInfo? ImportInfos(DatCollection dats, uint layoutId)
     {
         var ld = dats.Get<LayoutDesc>(layoutId);