feat(D.5.1): faithful item-icon type-default underlay (EnumIDMap 0x10000004) — opaque icon backing

Retail IconData::RenderIcons (decomp 407524) builds the icon layer stack bottom→top:
type-default underlay (OPAQUE, Blit_Normal) first, then custom underlay, base icon,
custom overlay.  acdream's IconComposer omitted the type-default underlay, leaving
filled toolbar slots with a transparent background.

Resolution via the two-level EnumIDMap chain that retail uses (DBCache::GetDIDFromEnum
0x413940): Portal.Header.MasterMapId (0x25000000) → master[0x10000004] → submap DID
(0x25000008) → submap[LSB(itemType)+1] → 0x06 RenderSurface underlay DID.  Golden
values confirmed against the live dats: MeleeWeapon→0x060011CB, Armor→0x060011CF,
Clothing→0x060011F3, Jewelry→0x060011D5, None(fallback 0x21)→0x060011D4.

Changes:
- IconComposer: add ResolveUnderlayDid(ItemType)/EnsureUnderlaySubMap (memoised);
  widen cache key from (uint,uint,uint)→(uint,uint,uint,uint); GetIcon gains ItemType
  param and prepends the opaque underlay as layer 0 (Compose sizes to it → fully opaque)
- ToolbarController: widen _iconIds Func from 3-arg to 4-arg; Populate passes item.Type
- GameWindow: update toolbar mount lambda to 4-arg form
- Tests: update ToolbarController test stubs to (_,_,_,_); add
  Compose_opaqueUnderlayFirst_resultIsFullyOpaque (dat-free) and
  ResolveUnderlayDid_goldenValues_matchDat (dat-gated, skip when dats absent)

No divergence-register row existed for this omission; none added (fully ported now).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/GameWindow.cs

@@ -1920,7 +1920,7 @@ public sealed class GameWindow : IDisposable
                 _toolbarController = AcDream.App.UI.Layout.ToolbarController.Bind(
                     toolbarLayout, Items,
                     () => Shortcuts,
-                    iconIds: (icon, under, over) => iconComposer.GetIcon(icon, under, over),
+                    iconIds: (type, icon, under, over) => iconComposer.GetIcon(type, icon, under, over),
                     useItem: guid => UseItemByGuid(guid),
                     combatState: Combat);