Add ChannelFader: an audio-taper gain fader over a level meter

[matt-edmondson]

Requested by Matt · project thread

Before: ktsu.ImGui.Widgets had no mixer-strip gain control. A caller who wanted one used a linear slider, so unity sat near the top and the handle and any meter beside it read on different scales.

After: ImGuiWidgets.ChannelFader draws a vertical gain fader over a level meter. The handle and the meter share one audio taper (FaderTaper), so a level and a gain at the same height mean the same number of decibels. You drag it, click the track to jump, use the wheel to nudge by 1 dB, and right-click to return to unity. It marks itself with ImGuiProbes.MarkItem.

How:

• FaderTaper maps decibels to travel. ChannelFaderState holds the interaction with no ImGui dependency and is unit tested without a context.
• MeterScale.DrawVerticalMeter gains an overload that takes a level-to-height mapping, so the meter is drawn on the fader's taper. The existing linear overload delegates to it, and its behaviour is unchanged.
• It ships with a ChannelFaderDemo registered with one line in DemoSections.cs, an isolation suite in ImGui.Widgets.UITests, and a gallery tile.

One deviation from #528: ChannelFaderImpl keeps a per-id DragPositions handle position across the frames of a drag. Rebuilding the position from gainDb every frame sends it through the taper and back, which differs in the last bit from where the drag left it. The fader would then report a change on every frame while the mouse is held still.

Closes #528

🤖 Generated with Claude Code

https://claude.ai/code/session_01MBhLQ3ZkoYk8s5tKQ8RTAP

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Generated by Claude Code

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Quality Gate passed

Issues
0 New issues
0 Accepted issues

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98.7% Coverage on New Code
0.0% Duplication on New Code

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