feat(D.2b): UiField (Type 3) — editable input as a generic field; remove the stray Type-12 input placeholder (widget-generalization Task 6)

- Rename UiChatInput → UiField (UIElement_Field, RegisterElementClass(3) @ :126190);
  update doc to cite retail's CatchDroppedItem/MouseOverTop drag-drop hooks for
  future item windows. BackgroundColor default → transparent (controller sets
  the translucent 0.35α value explicitly, matching UiText pattern).
- Register Type 3 in DatWidgetFactory.Create: `3 => new UiField()`.
- ChatWindowController.Bind (Variant B): factory now builds 0x10000016 as an
  invisible UiText placeholder (Type 12); Bind removes that placeholder via
  FindElement(InputId).Parent.RemoveChild and places a UiField at the same rect.
  Result: exactly ONE input widget in the input bar, no stray UiText duplicate.
- Input property type changed from UiChatInput to UiField; GameWindow.cs:1861
  UiField.Keyboard assignment compiles unchanged (field exists).
- Tests: UiChatInputTests → UiFieldTests (class + all ctor refs renamed);
  DatWidgetFactoryTests: new Type3_Field_MakesUiField test; ChatWindowControllerTests:
  updated stale "skipped by factory" comments; LayoutConformanceTests: updated
  VitalsTree_ChromeCornerHasExpectedSprite — Type-3 chrome-corner elements are
  now UiField (sprite rendering for Type-3 dat image elements is a known
  limitation, tracked for post-Task-8 UiField.BackgroundSprite follow-up).
- Full suite: 404 passed, 2 skipped, 0 failed.

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

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

@@ -52,6 +52,7 @@ public static class DatWidgetFactory
         UiElement e = info.Type switch
         {
             1  => new UiButton(info, resolve),         // UIElement_Button (reg :125828)
+            3  => new UiField(),                       // UIElement_Field (reg :126190)
             6  => new UiMenu(),                        // UIElement_Menu (reg :120163)
             7  => BuildMeter(info, resolve, datFont),  // UIElement_Meter
             11 => new UiScrollbar(),                   // UIElement_Scrollbar (reg :124137)