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.Abstractions/IPanelRenderer.cs

@@ -162,4 +162,40 @@ public interface IPanelRenderer
     /// Use for descriptions, error messages, anything multi-line.
     /// </summary>
     void TextWrapped(string text);
+
+    // -- Phase J Tier 3 — scrollable child region for chat-style layouts --
+
+    /// <summary>
+    /// Open a scrollable nested region inside the current window.
+    /// <paramref name="size"/> follows ImGui semantics: 0 means "fill
+    /// available", a positive value is absolute pixels, a negative
+    /// value means "fill available minus this amount". Pass
+    /// <c>(0, -footerHeight)</c> to reserve space at the bottom for
+    /// fixed elements (separator + input field) so the inner content
+    /// scrolls but the footer stays put across window resizes.
+    /// </summary>
+    bool BeginChild(string id, Vector2 size, bool border = false);
+
+    /// <summary>Close the most recent <see cref="BeginChild"/>.</summary>
+    void EndChild();
+
+    /// <summary>
+    /// Approximate single-line widget height including ImGui's frame
+    /// padding + item spacing. Panels use this to compute footer
+    /// reservations for <see cref="BeginChild"/> (e.g. one input
+    /// field + spacing). Backend-friendly because it returns a
+    /// scalar that the custom retail-look toolkit can also expose.
+    /// </summary>
+    float FrameHeightWithSpacing();
+
+    /// <summary>
+    /// Scroll the current scroll region so that the given fractional
+    /// vertical position is visible, where 0 = top and 1 = bottom.
+    /// Typical usage: call with <c>1.0f</c> at the bottom of a
+    /// chat-tail render block to keep the latest line visible when
+    /// new entries arrive. No-op when no new content was emitted —
+    /// callers are expected to skip the call when they don't want
+    /// to force a scroll.
+    /// </summary>
+    void SetScrollHereY(float ratio);
 }