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>
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