fix(D.2b): Attributes tab — fill panel height, center row icons, footer at bottom

Root cause: the sub-layout 0x2100002C (design H=337px) mounted into tab slot
0x1000022B (H=575px) via ShouldMountBaseChildren. ElementReader.Merge takes the
derived (sub-layout) H=337 as canonical, so the background element's first
ApplyAnchor call captured _amB=238 and the list box captured _amB=65 — both
stayed at their 337px-parent sizes even though the slot is 575px.

Fix: CascadeHeight in LayoutImporter.Resolve mirrors retail's
UIElement::UpdateForParentSizeChange. When ShouldMountBaseChildren fires and the
slot is taller than the base design, every full-stretch background child has its
height cascaded: Top+Bottom anchors → stretch, Bottom-only → pin-to-bottom,
Top-only/None → unchanged. This propagates the correct bottom margins through the
entire subtree before the first render frame.

Layout result (confirmed via headless screenshot):
- List box grows from 160px → 398px (9 rows × 44px ≈ 396px)
- Footer elements move from abs Y≈307 → abs Y≈545 (matching retail dump)
- Separator moves to abs Y≈535

Row constants updated: RowHeight 44px, IconSize 24px, RowPadX 4px, IconGap 6px.
Footer State-A text corrected per spec: title="Select an Attribute to Improve",
line-1 label="Skill Credits Available:", line-1 value=SkillCredits (96),
line-2 label="Unassigned Experience:", line-2 value=UnassignedXp (formatted N0).
CharacterSheet.UnassignedXp (long, retail InqInt64(2)) added.
SampleData.SampleCharacter().UnassignedXp = 87_757_321_741L.
5 footer tests renamed + assertions updated; SampleCharacter_UnassignedXp_IsSet added.
639 tests green.

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

src/AcDream.App/Studio/FixtureProvider.cs

@@ -152,4 +152,5 @@ public static class FixtureProvider
             var (handle, _, _) = stack.ResolveChrome(iconId);
             return handle;
         };
+
 }

src/AcDream.App/Studio/SampleData.cs

@@ -157,6 +157,9 @@ public static class SampleData
         // Available skill credits (retail InqInt(0x18)); shown in Attributes tab footer State-A.
         SkillCredits = 96,
 
+        // Unassigned (banked) XP (retail InqInt64(2)); footer State-A line-2 value.
+        UnassignedXp = 87_757_321_741L,
+
         AugmentationName = "Swords",
 
         BurdenCurrent = 1200,