feat(ui): Character tab — per-toon settings; Phase L.0 complete

Phase L.0 (final) — last tab on the Settings shell. Per-toon
preferences keyed by toon name in settings.json under
character[<toonName>]. With this commit the L.0 build order
finishes and every approved tab is implemented.

CharacterSettings record (4 fields):
 · DefaultChatChannel (string — Local / Allegiance / Fellowship / etc)
 · AutoAttack (bool — continue swinging until target dies)
 · ConfirmSalvage (bool — prompt before salvaging valuable items)
 · ShowPickupMessages (bool — pickup lines in chat)

AvailableChannels static list exposes the 7 retail-routing targets
for the dropdown.

SettingsStore grows LoadCharacter(toonKey) / SaveCharacter(toonKey)
using JsonNode/JsonObject for the nested-toon write — the existing
SaveSection raw-text-preservation pattern handles top-level keys
but doesn't fit the nested per-toon mutation. The character map
preserves every other toon's settings on save, and other top-level
sections (display / audio / gameplay / chat) are preserved too.

SettingsVM grows the parallel character state machine. The host
owns the toonKey (currently hard-coded to "default" in GameWindow
because we don't have a current-character source plumbed yet) —
the VM just edits whatever bag the host loaded.

SettingsPanel.RenderCharacterTab replaces the L.0-shell placeholder
— a Combo for default chat channel + 3 Checkboxes for
AutoAttack / ConfirmSalvage / ShowPickupMessages. The
RenderPlaceholder helper is now removed (no callers); the old
"Placeholder_tabs_render_coming_soon_text_when_active" test is
replaced by an "all six tabs are implemented" guard test that
fails if any future commit adds a placeholder back.

GameWindow loads/saves character settings under toonKey "default"
with a TODO comment to swap in the real toon name once
CharacterList plumbing exposes a currentCharacter source.

18 new tests:
 · CharacterSettings record (4) — defaults pinned, AvailableChannels
   list shape, value equality, with-expressions
 · SettingsStore character (6) — missing-file / toon-not-in-file →
   defaults, round-trip, multi-toon preservation, preserves other
   top-level sections, all five sections coexist
 · SettingsVM character (5) — initial draft, SetCharacter marks
   dirty, Save invokes callback, Cancel reverts, ResetAllToDefaults
   covers
 · SettingsPanel character tab (3 net, after removing the
   placeholder test) — combo+checkboxes render only when active,
   channel combo uses AvailableChannels, all six tabs are now
   non-placeholder

Phase L.0 final tally:
 · 5 commits on feature/settings-retail (shell + 5 tabs)
 · 6 tabs: Keybinds (Phase K) + Display + Audio + Gameplay + Chat + Character
 · 5 settings sections in settings.json (display/audio/gameplay/chat/character),
   coexisting non-destructively + a sixth file (keybinds.json) on the side.

dotnet build green (0 warnings); dotnet test 1,307 / 1,307 green
(243 Core.Net + 391 UI.Abstractions + 673 Core).

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

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

@@ -0,0 +1,48 @@
+using AcDream.UI.Abstractions.Panels.Settings;
+
+namespace AcDream.UI.Abstractions.Tests.Panels.Settings;
+
+/// <summary>L.0: <see cref="CharacterSettings"/> default-pin tests.</summary>
+public sealed class CharacterSettingsTests
+{
+    [Fact]
+    public void Default_values_are_conservative()
+    {
+        var d = CharacterSettings.Default;
+        Assert.Equal("Local", d.DefaultChatChannel);
+        Assert.False(d.AutoAttack);
+        Assert.True(d.ConfirmSalvage);
+        Assert.True(d.ShowPickupMessages);
+    }
+
+    [Fact]
+    public void AvailableChannels_includes_retail_routing_targets()
+    {
+        var list = CharacterSettings.AvailableChannels;
+        Assert.Contains("Local", list);
+        Assert.Contains("Allegiance", list);
+        Assert.Contains("Fellowship", list);
+        Assert.Contains("General", list);
+        Assert.Contains("Trade", list);
+        Assert.Contains("LFG", list);
+        Assert.Contains("Roleplay", list);
+    }
+
+    [Fact]
+    public void Equality_is_value_based()
+    {
+        var a = CharacterSettings.Default;
+        var b = CharacterSettings.Default with { AutoAttack = true };
+        var c = CharacterSettings.Default with { AutoAttack = true };
+        Assert.NotEqual(a, b);
+        Assert.Equal(b, c);
+    }
+
+    [Fact]
+    public void With_expression_clones_one_field()
+    {
+        var d = CharacterSettings.Default with { DefaultChatChannel = "Allegiance" };
+        Assert.Equal("Allegiance", d.DefaultChatChannel);
+        Assert.False(d.AutoAttack);
+    }
+}