feat(ui): tabbed Settings shell — IPanelRenderer tab API + 6 placeholder tabs

Phase L.0 — foundation for the complete retail-style Settings interface
agreed in the 2026-04-26 brainstorm. Splits Phase K's keybind-only F11
panel into a tabbed shell whose first tab wraps the existing keybinds
content unchanged; the other five tabs (Display / Audio / Gameplay /
Chat / Character) render "Coming soon" placeholders so the shape the
user approved is visible immediately and gets filled in over the L.x
sub-phases (Display first per Easy-wins build order).

Why a tab API extension: retail had distinct Options UIs
(gmGameplayOptionsUI / gmChatOptionsUI / gmCharacterSettingsUI per the
PDB at acclient_2013_pseudo_c.txt:170739+) and the existing
IPanelRenderer only exposed CollapsingHeader. ImGui maps
BeginTabBar / BeginTabItem / EndTabItem / EndTabBar 1:1, so the new
primitives stay backend-friendly — the future D.2b custom retail-look
backend implements them via the retail tab UIs without panel changes.

Save / Cancel / Reset-all stay above the tab bar so they remain global
across all tabs (Phase K's UX preserved). FakePanelRenderer grows
matching tab calls + an ActiveTabLabel knob so tests can target a
specific tab's content; default behavior treats the first tab item
seen as active so existing tests keep passing without changes.

5 new SettingsPanelTests assertions: tab bar opens once, six expected
tab labels emitted in order, Keybinds-tab section headers only render
when active, placeholders show "Coming soon" text on inactive-content
tabs, and Save/Cancel buttons render BEFORE the tab bar (regression
guard against accidentally moving them inside a tab item).

dotnet build green (0 warnings); dotnet test 1,227 / 1,227 green
(243 Core.Net + 311 UI.Abstractions + 673 Core).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.UI.Abstractions/IPanelRenderer.cs

@@ -235,4 +235,31 @@ public interface IPanelRenderer
     /// frame the user clicks the item; false otherwise.
     /// </summary>
     bool MenuItem(string label, string? shortcut = null);
+
+    // -- Tab bar (Settings panel + future tabbed surfaces) ---------------
+
+    /// <summary>
+    /// Open a tab bar inside the current window. Returns <c>true</c>
+    /// when the bar is visible — only emit <see cref="BeginTabItem"/>
+    /// calls inside that branch. Always pair with
+    /// <see cref="EndTabBar"/> when the call returned true. Retail had
+    /// tab bars in the Options UIs (<c>gmGameplayOptionsUI</c> etc), so
+    /// this primitive must be expressible by the future custom
+    /// retail-look backend.
+    /// </summary>
+    bool BeginTabBar(string id);
+
+    /// <summary>Close the tab bar opened by <see cref="BeginTabBar"/>.</summary>
+    void EndTabBar();
+
+    /// <summary>
+    /// Begin a single tab inside an open <see cref="BeginTabBar"/>.
+    /// Returns <c>true</c> when the tab is the currently selected one
+    /// — only render this tab's content in that branch. Always pair
+    /// with <see cref="EndTabItem"/> when the call returned true.
+    /// </summary>
+    bool BeginTabItem(string label);
+
+    /// <summary>Close the tab opened by <see cref="BeginTabItem"/>.</summary>
+    void EndTabItem();
 }

src/AcDream.UI.Abstractions/Panels/Settings/SettingsPanel.cs

@@ -5,25 +5,23 @@ using AcDream.UI.Abstractions.Input;
 namespace AcDream.UI.Abstractions.Panels.Settings;
 
 /// <summary>
-/// K.3: in-game Settings panel for click-to-rebind keymap editing.
-/// Hidden by default; opens via <c>F11</c> (which fires the
-/// <see cref="InputAction.ToggleOptionsPanel"/> action) or via the
-/// View → Settings entry on the main menu bar.
+/// In-game Settings panel — F11 toggle (or View → Settings on the main
+/// menu bar). Hidden by default. Tabbed: Keybinds (Phase K), then
+/// Display / Audio / Gameplay / Chat / Character (filling in over the
+/// L.x sub-phases).
 ///
 /// <para>
-/// Layout: top row of action buttons (Save / Cancel / Reset all), then
-/// a sequence of <see cref="IPanelRenderer.CollapsingHeader"/> sections
-/// matching the retail keymap categories (Movement / Postures / Camera /
-/// Combat / UI panels / Chat / Hotbar / Emotes). Each row inside a
-/// section: action name, current binding(s) summary, "Rebind" button,
-/// per-action "Reset" button. When a rebind is in progress the Rebind
-/// button label changes to "Press a key... (Esc to cancel)".
+/// Top of the panel: Save / Cancel / Reset-all action buttons (global
+/// across all tabs). When <see cref="SettingsVM.PendingConflict"/> is
+/// non-null, a confirmation prompt is rendered above those buttons
+/// (Yes — Reassign / No — Keep existing).
 /// </para>
 ///
 /// <para>
-/// When <see cref="SettingsVM.PendingConflict"/> is non-null, a
-/// confirmation prompt is rendered ABOVE the rest of the panel (Yes —
-/// Reassign / No — Keep existing).
+/// Below the action row a tab bar selects between the six categories.
+/// Only the Keybinds tab is implemented today; the other five render
+/// "Coming soon" placeholders so the structure the user approved in the
+/// design brainstorm is visible immediately.
 /// </para>
 /// </summary>
 public sealed class SettingsPanel : IPanel
@@ -42,7 +40,7 @@ public sealed class SettingsPanel : IPanel
     public string Title => "Settings";
 
     /// <inheritdoc />
-    /// <remarks>K.3: hidden by default — opened via F11 / View menu.</remarks>
+    /// <remarks>Hidden by default — opened via F11 / View menu.</remarks>
     public bool IsVisible { get; set; } = false;
 
     /// <inheritdoc />
@@ -67,7 +65,7 @@ public sealed class SettingsPanel : IPanel
             renderer.Separator();
         }
 
-        // Top action buttons.
+        // Top action buttons. Global across all tabs.
         if (renderer.Button("Save changes")) _vm.Save();
         renderer.SameLine();
         if (renderer.Button("Cancel changes")) _vm.Cancel();
@@ -76,7 +74,51 @@ public sealed class SettingsPanel : IPanel
 
         renderer.Separator();
 
-        // Sections (retail keymap categories).
+        if (renderer.BeginTabBar("settings.tabs"))
+        {
+            if (renderer.BeginTabItem("Keybinds"))
+            {
+                RenderKeybindsTab(renderer);
+                renderer.EndTabItem();
+            }
+            if (renderer.BeginTabItem("Display"))
+            {
+                RenderPlaceholder(renderer, "Display");
+                renderer.EndTabItem();
+            }
+            if (renderer.BeginTabItem("Audio"))
+            {
+                RenderPlaceholder(renderer, "Audio");
+                renderer.EndTabItem();
+            }
+            if (renderer.BeginTabItem("Gameplay"))
+            {
+                RenderPlaceholder(renderer, "Gameplay");
+                renderer.EndTabItem();
+            }
+            if (renderer.BeginTabItem("Chat"))
+            {
+                RenderPlaceholder(renderer, "Chat");
+                renderer.EndTabItem();
+            }
+            if (renderer.BeginTabItem("Character"))
+            {
+                RenderPlaceholder(renderer, "Character");
+                renderer.EndTabItem();
+            }
+            renderer.EndTabBar();
+        }
+
+        renderer.End();
+    }
+
+    /// <summary>
+    /// Render the Keybinds tab — eight collapsing-header sections matching
+    /// the retail keymap categories. Phase K shipped this content; the
+    /// only thing that changed is the wrapping tab item.
+    /// </summary>
+    private void RenderKeybindsTab(IPanelRenderer renderer)
+    {
         RenderSection(renderer, "Movement", new[]
         {
             InputAction.MovementForward, InputAction.MovementBackup,
@@ -136,8 +178,20 @@ public sealed class SettingsPanel : IPanel
             InputAction.Cry, InputAction.Laugh, InputAction.Wave,
             InputAction.Cheer, InputAction.PointState,
         });
+    }
 
-        renderer.End();
+    /// <summary>
+    /// Placeholder content shown for tabs whose implementation is still
+    /// pending. Reads as "Coming soon" plus a note about which sub-phase
+    /// is expected to fill it in.
+    /// </summary>
+    private static void RenderPlaceholder(IPanelRenderer renderer, string tabName)
+    {
+        renderer.TextWrapped($"{tabName} settings coming soon.");
+        renderer.Spacing();
+        renderer.TextWrapped(
+            "This tab is part of the staged Settings interface rollout. "
+            + "Build order: Display → Audio → Gameplay → Chat → Character.");
     }
 
     private void RenderSection(IPanelRenderer renderer, string label, InputAction[] actions)

src/AcDream.UI.ImGui/ImGuiPanelRenderer.cs

@@ -193,4 +193,18 @@ public sealed class ImGuiPanelRenderer : IPanelRenderer
         => shortcut is null
             ? ImGuiNET.ImGui.MenuItem(label)
             : ImGuiNET.ImGui.MenuItem(label, shortcut);
+
+    // -- Tab bar -----------------------------------------------------------
+
+    /// <inheritdoc />
+    public bool BeginTabBar(string id) => ImGuiNET.ImGui.BeginTabBar(id);
+
+    /// <inheritdoc />
+    public void EndTabBar() => ImGuiNET.ImGui.EndTabBar();
+
+    /// <inheritdoc />
+    public bool BeginTabItem(string label) => ImGuiNET.ImGui.BeginTabItem(label);
+
+    /// <inheritdoc />
+    public void EndTabItem() => ImGuiNET.ImGui.EndTabItem();
 }