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/UI/Layout/CharacterSheet.cs

@@ -94,10 +94,17 @@ public sealed class CharacterSheet
     /// <summary>
     /// Available (unspent) skill credits shown in the Attributes tab footer State-A.
     /// Retail InqInt(0x18) — gmStatManagementUI::DisplayDefaultFooter (0x0049cde0).
-    /// Element 0x10000245 (footer line-2 value).
+    /// Element 0x10000243 (footer line-1 value in the studio's 3-line layout).
     /// </summary>
     public int SkillCredits { get; init; }
 
+    /// <summary>
+    /// Unassigned (banked) experience points.
+    /// Retail InqInt64(2) — shown in footer line-2 in State-A display.
+    /// Element 0x10000245 (footer line-2 value).
+    /// </summary>
+    public long UnassignedXp { get; init; }
+
     // ── Augmentations  (UpdateAugmentations 0x004b9000) ─────────────────────
     // Retail InqInt(0x162) = AugmentationStat; string-switch 1..0xb.