feat(ui): chat Copy mode — select + Ctrl+C any text in the chat tail

User reported wanting to mark text in-game and copy it out (item names,
coordinates, NPC dialogue, etc). ImGui doesn't natively let you select
across multiple TextColored widgets, but a read-only multi-line
InputText is fully click-drag selectable + Ctrl+C copyable. This
commit adds a "Copy mode" toggle to ChatPanel that swaps the chat
tail's render path between the colored-line view and a single
selectable text region.

New IPanelRenderer primitive:

  void TextMultilineReadOnly(string id, string content, Vector2 size);

ImGui maps this to InputTextMultiline with the ReadOnly flag — same
selection + Ctrl+C UX a user expects from any text-input widget.
FakePanelRenderer records the call for tests. The future D.2b
custom retail-look backend implements its own equivalent (likely
the same widget pattern with retail font/skin).

ChatPanel rendering:

  · A "Copy mode (select text to Ctrl+C)" Checkbox at the top of
    the panel toggles _copyMode.
  · Off (default) — current per-line render with colored combat
    entries. Visually unchanged from before.
  · On — the chat tail becomes a single TextMultilineReadOnly
    widget holding every visible line joined with newlines. Loses
    per-line color, gains arbitrary-span text selection.
  · Footer (separator + input field) renders identically in both
    modes so the user can still type while in copy mode.

Existing ChatPanelLayoutTests's footer-separator probe was using
IndexOf("Separator") — which now matches the new pre-tail separator
between the Checkbox and the chat tail. Switched to LastIndexOf
which still pins the footer separator (between EndChild and
InputTextSubmit). Behaviour and intent unchanged.

DisplaySettingsTests' With_expression test was still asserting the
old "1920x1080" Default.Resolution; updated to the new "1280x720"
that the previous wire-up commit introduced (the earlier commit
forgot this one).

dotnet build green (0 warnings); dotnet test 1,309 / 1,309 green
(243 Core.Net + 393 UI.Abstractions + 673 Core).

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

src/AcDream.UI.Abstractions/IPanelRenderer.cs

@@ -262,4 +262,21 @@ public interface IPanelRenderer
 
     /// <summary>Close the tab opened by <see cref="BeginTabItem"/>.</summary>
     void EndTabItem();
+
+    /// <summary>
+    /// Render a read-only multi-line text region the user can
+    /// <b>select</b> with click+drag and copy with <c>Ctrl+C</c>.
+    /// Matches the typical "click into a textbox to grab text" UX —
+    /// chat panels, log viewers, etc. use this to make text
+    /// extractable without the user having to alt-tab + retype.
+    ///
+    /// <para>
+    /// The widget is sized to <paramref name="size"/>; pass
+    /// <c>(0, 0)</c> for "fill the current content region" semantics
+    /// (matches ImGui defaults). <paramref name="id"/> is the ImGui
+    /// stable identifier — typically <c>"##chatcopy"</c> or similar
+    /// hidden-label form.
+    /// </para>
+    /// </summary>
+    void TextMultilineReadOnly(string id, string content, Vector2 size);
 }

src/AcDream.UI.Abstractions/Panels/Chat/ChatPanel.cs

@@ -1,3 +1,4 @@
+using System.Linq;
 using System.Numerics;
 using AcDream.Core.Chat;
 using AcDream.Core.Combat;
@@ -50,6 +51,13 @@ public sealed class ChatPanel : IPanel
     // click into another widget.
     private bool _focusRequested;
 
+    // L.0 follow-up: "Copy mode" — when true, render the chat tail as
+    // a read-only multi-line text widget the user can click+drag to
+    // select + Ctrl+C to copy. Trades per-line color for selectability;
+    // user toggles when they want to grab specific text out of the
+    // log (item names, coordinates, NPC dialogue, etc).
+    private bool _copyMode;
+
     public ChatPanel(ChatVM vm)
     {
         _vm = vm ?? throw new ArgumentNullException(nameof(vm));
@@ -82,6 +90,17 @@ public sealed class ChatPanel : IPanel
             return;
         }
 
+        // L.0 follow-up: top-of-panel "Copy mode" toggle. When on, the
+        // chat tail rendering swaps to TextMultilineReadOnly so the
+        // user can mark + Ctrl+C any text. Off (default) preserves the
+        // colored per-line render with combat highlights. The checkbox
+        // sits ABOVE the chat tail (not in the footer) so it's always
+        // visible regardless of scroll position.
+        bool copyMode = _copyMode;
+        if (renderer.Checkbox("Copy mode (select text to Ctrl+C)", ref copyMode))
+            _copyMode = copyMode;
+        renderer.Separator();
+
         // Phase J Tier 3: keep the input field at the bottom of the
         // window across resizes by reserving footer space and putting
         // the chat tail in a scrollable child that fills the rest.
@@ -95,7 +114,21 @@ public sealed class ChatPanel : IPanel
         // the plain Text path (visually identical to the I.4 panel).
         var lines = _vm.RecentLinesDetailed();
 
-        if (renderer.BeginChild("##chattail", new System.Numerics.Vector2(0, -footerHeight)))
+        if (_copyMode)
+        {
+            // Copy mode: one big read-only multiline text widget
+            // holding every visible line, joined with newlines. Loses
+            // per-line color but lets the user click+drag to select
+            // arbitrary spans of text + Ctrl+C to copy. Sized to fill
+            // the available space minus the footer.
+            string joined = lines.Count == 0
+                ? "(no messages yet)"
+                : string.Join("\n", lines.Select(l => l.Text));
+            renderer.TextMultilineReadOnly(
+                "##chattailcopy", joined,
+                new System.Numerics.Vector2(0f, -footerHeight));
+        }
+        else if (renderer.BeginChild("##chattail", new System.Numerics.Vector2(0, -footerHeight)))
         {
             if (lines.Count == 0)
             {
@@ -127,7 +160,7 @@ public sealed class ChatPanel : IPanel
             }
             _lastRenderedCount = lines.Count;
         }
-        renderer.EndChild();
+        if (!_copyMode) renderer.EndChild();
 
         // Phase I.4: input field. Backend implementation clears _input
         // on submit per the IPanelRenderer contract.

src/AcDream.UI.ImGui/ImGuiPanelRenderer.cs

@@ -207,4 +207,21 @@ public sealed class ImGuiPanelRenderer : IPanelRenderer
 
     /// <inheritdoc />
     public void EndTabItem() => ImGuiNET.ImGui.EndTabItem();
+
+    // -- Selectable / copyable text ---------------------------------------
+
+    /// <inheritdoc />
+    public void TextMultilineReadOnly(string id, string content, Vector2 size)
+    {
+        // ImGui's InputTextMultiline takes a `ref string` even with the
+        // ReadOnly flag — we just hand it a local copy. maxLength caps
+        // what the user could type if ReadOnly were ever cleared; we
+        // size it to the current content (+1 for ImGui's internal NUL
+        // terminator in some bindings). Min of 1 keeps the empty case
+        // from confusing native bindings.
+        string buffer = content;
+        uint maxLen = (uint)System.Math.Max(content.Length + 1, 1);
+        ImGuiNET.ImGui.InputTextMultiline(id, ref buffer, maxLen, size,
+            ImGuiInputTextFlags.ReadOnly);
+    }
 }