feat(D.2b): UiMenu (Type 6) — generic dropdown; channel knowledge moves to controller (widget-generalization Task 4)

UiChannelMenu → UiMenu: removed ChatChannelKind, the 14-item array, the
button-text map, and the availability default. Generic surface: MenuItem
(label + object? Payload), Selected (object?), OnSelect, EnabledProvider,
ButtonLabelProvider, RowsPerColumn/RowHeight/ColumnWidth (all settable).
All draw/event mechanics unchanged — same popup geometry, same click
coordinates, same 8-piece bevel, same 3-slice button face.

ChatWindowController gains ChannelItems[], ChannelButtonLabel(), and
ChannelAvailable() (verbatim from old widget), and populates the
factory-built Type-6 UiMenu via find-by-id rather than constructing a
replacement widget. The Menu property type is now UiMenu. OnChannelChanged
wrap replaced with the generic OnSelect wrap for the ReflowInputRow hook.

DatWidgetFactory registers Type 6 → new UiMenu().

Tests: UiChannelMenuTests → UiMenuTests (10 tests, all green); factory
Type6 test added; ChatWindowControllerTests updated to use OnSelect.
Divergence register: AP-42 added (flat item model vs retail nested-submenu
MakePopup @0x46d310 — latent, unreachable through the chat menu).

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

tests/AcDream.App.Tests/UI/Layout/DatWidgetFactoryTests.cs

@@ -115,6 +115,15 @@ public class DatWidgetFactoryTests
         Assert.IsType<UiScrollbar>(e);
     }
 
+    // ── Test 5d: Type 6 → UiMenu ─────────────────────────────────────────────
+
+    [Fact]
+    public void Type6_Menu_MakesUiMenu()
+    {
+        var e = DatWidgetFactory.Create(new ElementInfo { Type = 6, Width = 46, Height = 18 }, NoTex, null);
+        Assert.IsType<UiMenu>(e);
+    }
+
     // ── Test 6: Meter slice extraction (the important one) ───────────────────
 
     /// <summary>