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>

tests/AcDream.UI.Abstractions.Tests/Panels/Settings/SettingsPanelTests.cs

@@ -165,4 +165,93 @@ public sealed class SettingsPanelTests
         var (panel, _, _, _) = Build();
         Assert.Equal("acdream.settings", panel.Id);
     }
+
+    // -- Tabbed shell -----------------------------------------------------
+
+    [Fact]
+    public void Render_opens_tab_bar_with_six_tab_items()
+    {
+        var (panel, _, _, _) = Build();
+        var r = new FakePanelRenderer();
+
+        panel.Render(new PanelContext(0.016f, new NullBus()), r);
+
+        // BeginTabBar exactly once, EndTabBar exactly once.
+        Assert.Single(r.Calls, c => c.Method == "BeginTabBar");
+        Assert.Single(r.Calls, c => c.Method == "EndTabBar");
+
+        // The six tab labels approved in the design brainstorm.
+        var tabLabels = r.Calls.Where(c => c.Method == "BeginTabItem")
+            .Select(c => (string)c.Args[0]!).ToList();
+        Assert.Equal(
+            new[] { "Keybinds", "Display", "Audio", "Gameplay", "Chat", "Character" },
+            tabLabels);
+    }
+
+    [Fact]
+    public void Keybinds_tab_renders_section_headers_when_active()
+    {
+        var (panel, _, _, _) = Build();
+        // Default ActiveTabLabel = null → FakePanelRenderer treats the
+        // first tab item ("Keybinds") as active.
+        var r = new FakePanelRenderer { CollapsingHeaderNextReturn = false };
+
+        panel.Render(new PanelContext(0.016f, new NullBus()), r);
+
+        var headers = r.Calls.Where(c => c.Method == "CollapsingHeader")
+            .Select(c => (string)c.Args[0]!).ToList();
+        Assert.Contains("Movement", headers);
+        Assert.Contains("Hotbar", headers);
+        Assert.Contains("Emotes", headers);
+    }
+
+    [Fact]
+    public void Inactive_tabs_do_not_render_keybind_section_headers()
+    {
+        var (panel, _, _, _) = Build();
+        // Force "Display" to be the active tab — the Keybinds content
+        // must NOT render.
+        var r = new FakePanelRenderer { ActiveTabLabel = "Display" };
+
+        panel.Render(new PanelContext(0.016f, new NullBus()), r);
+
+        var headers = r.Calls.Where(c => c.Method == "CollapsingHeader")
+            .Select(c => (string)c.Args[0]!).ToList();
+        Assert.DoesNotContain("Movement", headers);
+        Assert.DoesNotContain("Hotbar", headers);
+    }
+
+    [Fact]
+    public void Placeholder_tabs_render_coming_soon_text_when_active()
+    {
+        var (panel, _, _, _) = Build();
+        var r = new FakePanelRenderer { ActiveTabLabel = "Audio" };
+
+        panel.Render(new PanelContext(0.016f, new NullBus()), r);
+
+        var wrapped = r.Calls.Where(c => c.Method == "TextWrapped")
+            .Select(c => (string)c.Args[0]!).ToList();
+        Assert.Contains(wrapped, t => t.Contains("Audio settings coming soon"));
+    }
+
+    [Fact]
+    public void Save_Cancel_buttons_render_outside_the_tab_bar()
+    {
+        // The global Save / Cancel / Reset-all row must come BEFORE
+        // BeginTabBar so it stays visible on every tab. Any change that
+        // accidentally moves the buttons inside a tab item should fail
+        // here.
+        var (panel, _, _, _) = Build();
+        var r = new FakePanelRenderer();
+
+        panel.Render(new PanelContext(0.016f, new NullBus()), r);
+
+        int saveIdx    = r.Calls.FindIndex(c => c.Method == "Button"
+            && (string)c.Args[0]! == "Save changes");
+        int tabBarIdx  = r.Calls.FindIndex(c => c.Method == "BeginTabBar");
+        Assert.True(saveIdx >= 0);
+        Assert.True(tabBarIdx >= 0);
+        Assert.True(saveIdx < tabBarIdx,
+            $"Save button (index {saveIdx}) must render before BeginTabBar (index {tabBarIdx}).");
+    }
 }