fix(ui): chat input pinned to window bottom on resize via scrollable child

User reported the chat input field disappearing when the chat
window was resized smaller — older entries pushed it past the
visible area. Standard ImGui chat-window pattern fixes it: scrollable
nested region for the chat tail, fixed footer for the
separator + input field below it.

IPanelRenderer extensions (Phase J Tier 3):
- BeginChild(string id, Vector2 size, bool border = false) — opens
  a nested scrollable region. Size follows ImGui semantics:
  0 = fill available, negative = fill available minus this much.
- EndChild() — closes the nested region.
- FrameHeightWithSpacing() — single-line widget height incl. frame
  padding + item spacing. Lets panels compute footer reservations
  without hardcoding pixel constants.
- SetScrollHereY(float ratio) — forces scroll within current region;
  pass 1.0f to keep the latest line visible after new entries
  arrive.

ImGuiPanelRenderer impls. ImGui.NET's BeginChild signature changed
across versions (third arg moved from `bool border` to
`ImGuiChildFlags`); we cast a numeric literal (0x01 = Border bit)
to sidestep the rename. FrameHeightWithSpacing maps to
ImGui.GetFrameHeightWithSpacing(); SetScrollHereY to ImGui.SetScrollHereY.

ChatPanel restructured:
- Reserves footer height = FrameHeightWithSpacing() + 6f (small pad
  for the separator above the input).
- Wraps the chat tail in BeginChild("##chattail", (0, -footer))
  so the inner region scrolls independently of the window.
- Tracks _lastRenderedCount across frames and calls SetScrollHereY(1f)
  only when new entries appended — manual scroll-up isn't fought
  against; new messages jump the view back down only when they
  actually arrive.
- Header Separator removed (the BeginChild border is enough).

FakePanelRenderer extended with the four new methods + recording.
4 new tests in ChatPanelLayoutTests pin the layout invariants:
- Render order: Begin → BeginChild → ... → EndChild → Separator
  → InputTextSubmit → End.
- BeginChild size has X=0 + negative Y at least matching the
  injected FrameHeightWithSpacingValue.
- SetScrollHereY fires when entries grow.
- SetScrollHereY does NOT fire when entries don't grow.

Solution total: 1067 green (243 Core.Net + 164 UI + 660 Core),
0 warnings.

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

src/AcDream.UI.ImGui/ImGuiPanelRenderer.cs

@@ -150,4 +150,24 @@ public sealed class ImGuiPanelRenderer : IPanelRenderer
 
     /// <inheritdoc />
     public void TextWrapped(string text) => ImGuiNET.ImGui.TextWrapped(text);
+
+    // -- Phase J Tier 3 — scrollable child for chat-style layouts ---------
+
+    /// <inheritdoc />
+    public bool BeginChild(string id, Vector2 size, bool border = false)
+        // ImGuiChildFlags has changed names across ImGui.NET versions
+        // (Border vs Borders); 0x01 is the stable bit value for "draw
+        // a border". Casting from a numeric literal sidesteps the
+        // version-skew without requiring a hard reference to either
+        // enum spelling.
+        => ImGuiNET.ImGui.BeginChild(id, size, (ImGuiChildFlags)(border ? 0x01 : 0));
+
+    /// <inheritdoc />
+    public void EndChild() => ImGuiNET.ImGui.EndChild();
+
+    /// <inheritdoc />
+    public float FrameHeightWithSpacing() => ImGuiNET.ImGui.GetFrameHeightWithSpacing();
+
+    /// <inheritdoc />
+    public void SetScrollHereY(float ratio) => ImGuiNET.ImGui.SetScrollHereY(ratio);
 }