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>

docs/architecture/retail-divergence-register.md

@@ -139,6 +139,7 @@ accepted-divergence entries (#96, #49, #50).
 | AP-39 | Chat lines carry one color per `ChatKind` (per-line solid color); retail `UIElement_Text` supports per-glyph styled runs (bold, different hue per segment) | `src/AcDream.App/UI/UiChatView.cs:13` | Retail glyph-run parsing lives inside keystone.dll with no PDB/decomp; per-line per-kind coloring is the correct tonal palette and covers all existing chat types | Chat lines retail renders with multiple colors or bold names (e.g. "PlayerName says: text") render as one flat color; subtle visual difference but functionally complete | `UIElement_Text` glyph-run styling (keystone.dll, no decomp) |
 | AP-40 | Single default translucency for the chat window chrome; no focused/unfocused opacity transition; dat font face/size taken from the vitals `vitalsDatFont` (same dat font, not a chat-specific size lookup) | `src/AcDream.App/Rendering/GameWindow.cs` (chatController binding line) | Retail fades the chat window to ~80% alpha when unfocused (`gmMainChatUI::UpdateAlpha @0x4cdea0`); the opacity animation deferred to the Plan-2 window-manager input integration; sharing `vitalsDatFont` is safe — retail uses the same AC-default font for both | The chat window is always fully opaque/same-font rather than subtly fading when idle; no wrong text, but the focused/unfocused breathing rhythm is absent | `gmMainChatUI::UpdateAlpha` @0x4cdea0; `UCF::SetAceFont @0x4d3940` |
 | AP-41 | Scrollbar thumb 3-slice cap fallback only: single-tile draw (`0x06004C63`) used only when `ThumbTopSprite`/`ThumbBotSprite` are unset; the chat controller passes all three cap ids so the 3-slice path is drawn in practice | `src/AcDream.App/UI/UiScrollbar.cs:35` | The fallback single-tile path is unreachable when caps are bound (chat controller always sets them); the 3-slice path is the active code path | Only if a future caller omits the cap ids will the fallback fire — no visual regression in the chat window | `UIElement_Scrollbar::UpdateLayout @0x4710d0`; cap sprites `0x06004C60` (top) + `0x06004C66` (bottom) from base layout `0x2100003E` |
+| AP-42 | `UiMenu` item model is flat (label + opaque payload, single-level popup); retail `UIElement_Menu::MakePopup @0x46d310` supports hierarchical nested submenus via recursive popup chain | `src/AcDream.App/UI/UiMenu.cs` | The chat talk-focus menu is single-level (14 rows, 2 columns, no submenu); hierarchy is latent and unreachable through the chat window — no behavioral difference in the current usage | A future menu with nested submenus would render flat (only the top-level items drawn, no drill-down) | `UIElement_Menu::MakePopup` @0x46d310 |
 
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