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.Abstractions/IPanelRenderer.cs

@@ -1,3 +1,5 @@
+using System.Numerics;
+
 namespace AcDream.UI.Abstractions;
 
 /// <summary>
@@ -40,4 +42,124 @@ public interface IPanelRenderer
     /// <paramref name="overlay"/> is optional text (e.g. <c>"54%"</c>) rendered on top.
     /// </summary>
     void ProgressBar(float fraction, float width, string? overlay = null);
+
+    // -- Phase I.1 widget extensions ----------------------------------
+
+    /// <summary>
+    /// Draw a single line of text in the supplied RGBA color.
+    /// Used for combat-event color-coding (damage red, heal green) and
+    /// any other surface where a glance distinguishes severity.
+    /// </summary>
+    void TextColored(Vector4 rgba, string text);
+
+    /// <summary>
+    /// Open / close section grouping inside a window. Returns
+    /// <c>true</c> when the section is currently expanded — only render
+    /// the section's contents in that branch.
+    /// </summary>
+    /// <param name="defaultOpen">If the user has never toggled the
+    /// header before, start in this state.</param>
+    bool CollapsingHeader(string label, bool defaultOpen = true);
+
+    /// <summary>
+    /// Push an expandable nested node. Pair every successful
+    /// <c>true</c> return with a matching <see cref="TreePop"/>;
+    /// when this returns <c>false</c> do NOT call <see cref="TreePop"/>.
+    /// </summary>
+    bool TreeNode(string label);
+
+    /// <summary>Pop a previously-pushed <see cref="TreeNode"/>.</summary>
+    void TreePop();
+
+    /// <summary>
+    /// A boolean toggle. Returns <c>true</c> on the frame the user
+    /// flipped the box (and updates <paramref name="value"/> in place);
+    /// returns <c>false</c> the rest of the time.
+    /// </summary>
+    bool Checkbox(string label, ref bool value);
+
+    /// <summary>
+    /// A clickable button. Returns <c>true</c> on the single frame the
+    /// user clicked it; <c>false</c> every other frame.
+    /// </summary>
+    bool Button(string label);
+
+    /// <summary>
+    /// A drop-down selector. Returns <c>true</c> on the frame the user
+    /// changed the selection (and updates <paramref name="selectedIndex"/>
+    /// to the new value); <c>false</c> otherwise.
+    /// </summary>
+    bool Combo(string label, ref int selectedIndex, string[] items);
+
+    /// <summary>
+    /// A horizontal slider clamped to <paramref name="min"/> /
+    /// <paramref name="max"/>. Returns <c>true</c> on frames where the
+    /// user dragged the value (and updates <paramref name="value"/>);
+    /// <c>false</c> when idle.
+    /// </summary>
+    bool SliderFloat(string label, ref float value, float min, float max);
+
+    /// <summary>
+    /// Time-series line graph (e.g. fps history). The active window is
+    /// <paramref name="values"/>[<paramref name="offset"/>..
+    /// <paramref name="offset"/>+<paramref name="count"/>] interpreted
+    /// as a ring buffer.
+    /// </summary>
+    /// <param name="label">Header text shown above the plot.</param>
+    /// <param name="values">Sample buffer (kept by the caller).</param>
+    /// <param name="count">Number of valid samples in the ring.</param>
+    /// <param name="offset">Index of the oldest sample in
+    /// <paramref name="values"/>.</param>
+    /// <param name="overlay">Optional centered overlay text (e.g.
+    /// <c>"60 fps"</c>).</param>
+    /// <param name="min">Optional fixed lower bound; null = autoscale.</param>
+    /// <param name="max">Optional fixed upper bound; null = autoscale.</param>
+    /// <param name="size">Optional pixel size; null = backend default.</param>
+    void PlotLines(
+        string label,
+        float[] values,
+        int count,
+        int offset = 0,
+        string? overlay = null,
+        float? min = null,
+        float? max = null,
+        Vector2? size = null);
+
+    /// <summary>
+    /// Begin a multi-column table. Pair every call with <see cref="EndTable"/>.
+    /// Inside the table, advance to each cell with
+    /// <see cref="TableNextColumn"/> before emitting widgets for it.
+    /// </summary>
+    void BeginTable(string id, int columns);
+
+    /// <summary>Move to the next cell of the active table.</summary>
+    void TableNextColumn();
+
+    /// <summary>Close the most recent <see cref="BeginTable"/>.</summary>
+    void EndTable();
+
+    /// <summary>
+    /// A single-line text input that submits on Enter. Returns
+    /// <c>true</c> on the frame the user pressed Enter; in that case
+    /// <paramref name="submitted"/> is the value entered and the
+    /// implementation clears <paramref name="buffer"/> for the next
+    /// frame. On every other frame returns <c>false</c> and
+    /// <paramref name="submitted"/> is null; the implementation may
+    /// still mutate <paramref name="buffer"/> as the user types.
+    /// </summary>
+    /// <param name="maxLen">Maximum characters the buffer may grow to.</param>
+    bool InputTextSubmit(string label, ref string buffer, int maxLen, out string? submitted);
+
+    /// <summary>Insert a small vertical gap.</summary>
+    void Spacing();
+
+    /// <summary>Reserve invisible space of the given size, useful for
+    /// layout-only padding.</summary>
+    void Dummy(Vector2 size);
+
+    /// <summary>
+    /// Draw text that wraps at the current content region width.
+    /// Use for descriptions, error messages, anything multi-line.
+    /// </summary>
+    void TextWrapped(string text);
 }