feat(ui): #14 IPanelRenderer widget extension - TextColored, Checkbox, Combo, InputTextSubmit, BeginTable, etc.

Adds 14 widget signatures to IPanelRenderer + ImGuiPanelRenderer impl:
TextColored, CollapsingHeader, TreeNode/TreePop, Checkbox, Button,
Combo, SliderFloat, PlotLines, BeginTable/TableNextColumn/EndTable,
InputTextSubmit (Enter-key submit), Spacing, Dummy, TextWrapped.

InputTextSubmit uses ImGuiInputTextFlags.EnterReturnsTrue and clears
the buffer + emits via `out submitted` on the frame Enter is pressed.
PlotLines passes `ref values[0]` with empty-array guard. CollapsingHeader
defaultOpen=true uses ImGuiTreeNodeFlags.DefaultOpen (= 0x20).

FakePanelRenderer test double records (Method, Args) tuples and
exposes knobs to drive ref/out values. 17 new tests dispatch through
IPanelRenderer (not the concrete fake) so tests fail to compile when
the interface itself lacks a method - real RED -> GREEN signal.

Tests: 26 -> 43 in UI.Abstractions.Tests. Total solution 881 green.
Foundation for Phase I.2 (DebugPanel) and I.4 (ChatPanel input field).

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

src/AcDream.UI.ImGui/ImGuiPanelRenderer.cs

@@ -39,4 +39,115 @@ public sealed class ImGuiPanelRenderer : IPanelRenderer
         var size = new Vector2(width, 0f);   // height 0 → ImGui picks based on font
         ImGuiNET.ImGui.ProgressBar(fraction, size, overlay ?? string.Empty);
     }
+
+    // -- Phase I.1 widget extensions ---------------------------------
+
+    /// <inheritdoc />
+    public void TextColored(Vector4 rgba, string text)
+        => ImGuiNET.ImGui.TextColored(rgba, text);
+
+    /// <inheritdoc />
+    public bool CollapsingHeader(string label, bool defaultOpen = true)
+        => ImGuiNET.ImGui.CollapsingHeader(
+            label,
+            defaultOpen ? ImGuiTreeNodeFlags.DefaultOpen : ImGuiTreeNodeFlags.None);
+
+    /// <inheritdoc />
+    public bool TreeNode(string label) => ImGuiNET.ImGui.TreeNode(label);
+
+    /// <inheritdoc />
+    public void TreePop() => ImGuiNET.ImGui.TreePop();
+
+    /// <inheritdoc />
+    public bool Checkbox(string label, ref bool value)
+        => ImGuiNET.ImGui.Checkbox(label, ref value);
+
+    /// <inheritdoc />
+    public bool Button(string label) => ImGuiNET.ImGui.Button(label);
+
+    /// <inheritdoc />
+    public bool Combo(string label, ref int selectedIndex, string[] items)
+        => ImGuiNET.ImGui.Combo(label, ref selectedIndex, items, items.Length);
+
+    /// <inheritdoc />
+    public bool SliderFloat(string label, ref float value, float min, float max)
+        => ImGuiNET.ImGui.SliderFloat(label, ref value, min, max);
+
+    /// <inheritdoc />
+    public void PlotLines(
+        string label,
+        float[] values,
+        int count,
+        int offset = 0,
+        string? overlay = null,
+        float? min = null,
+        float? max = null,
+        Vector2? size = null)
+    {
+        // ImGui.NET 1.91.6.1's PlotLines binding takes `ref float values`
+        // (pointer-to-first-element semantics) plus a separate values_count
+        // and values_offset. The "no fixed bound" / "default size" sentinels
+        // are float.MaxValue and Vector2.Zero respectively — we pass those
+        // when the caller leaves the optional args null.
+        if (count <= 0 || values.Length == 0)
+        {
+            // Nothing to plot — emit the label so layout doesn't shift but
+            // skip the native call (ref to values[0] would NRE on empty).
+            ImGuiNET.ImGui.TextUnformatted(label);
+            return;
+        }
+
+        float scaleMin = min ?? float.MaxValue;
+        float scaleMax = max ?? float.MaxValue;
+        Vector2 graphSize = size ?? Vector2.Zero;
+        ImGuiNET.ImGui.PlotLines(
+            label,
+            ref values[0],
+            count,
+            offset,
+            overlay ?? string.Empty,
+            scaleMin,
+            scaleMax,
+            graphSize);
+    }
+
+    /// <inheritdoc />
+    public void BeginTable(string id, int columns)
+        => ImGuiNET.ImGui.BeginTable(id, columns);
+
+    /// <inheritdoc />
+    public void TableNextColumn() => ImGuiNET.ImGui.TableNextColumn();
+
+    /// <inheritdoc />
+    public void EndTable() => ImGuiNET.ImGui.EndTable();
+
+    /// <inheritdoc />
+    public bool InputTextSubmit(string label, ref string buffer, int maxLen, out string? submitted)
+    {
+        // EnterReturnsTrue: the call returns true on the frame the user
+        // pressed Enter. On every other frame ImGui still mutates `buffer`
+        // as the user types; we just don't surface a submit.
+        bool entered = ImGuiNET.ImGui.InputText(
+            label,
+            ref buffer,
+            (uint)maxLen,
+            ImGuiInputTextFlags.EnterReturnsTrue);
+        if (entered)
+        {
+            submitted = buffer;
+            buffer = string.Empty;   // contract: clear for next frame
+            return true;
+        }
+        submitted = null;
+        return false;
+    }
+
+    /// <inheritdoc />
+    public void Spacing() => ImGuiNET.ImGui.Spacing();
+
+    /// <inheritdoc />
+    public void Dummy(Vector2 size) => ImGuiNET.ImGui.Dummy(size);
+
+    /// <inheritdoc />
+    public void TextWrapped(string text) => ImGuiNET.ImGui.TextWrapped(text);
 }