feat(audio): the segment meter control (unit 3 of the dock)

`Controls/SegmentMeter` draws a 24-segment ladder in ONE OnRender pass — no per-segment elements
updating on the UI thread, which is the difference between a meter and a UI thread tax. Level and
peak are display fractions (what was MEASURED), never gain settings; vertical for the bay ladders,
horizontal for the master strip.

Bands come from `Helpers/MeterScale` (nominal/caution/peak, DESIGN.md thresholds) and live as
dependencies of the control, so the painting logic is pure and testable: the band a segment paints
is the level it measured. Brushes resolve from the theme tokens via SetResourceReference, so the
ladder cannot drift from DESIGN.md.

Peak-hold is the reason the header exists: a bar that follows the moment hides clipping, and
clipping is the one audio fault that cannot be fixed after the stream.

Guard: MeterScaleTests — band edges (inclusive at −12 and −3, as the standard states the ranges),
the −60…0 display scale, and peak-hold rising instantly then decaying at 0.35/s.

ytLive.Tests/MeterScaleTests.cs

@@ -0,0 +1,35 @@
+using Xunit;
+using ytLive.Helpers;
+
+namespace ytLive.Tests;
+
+/// <summary>A segment ladder is only useful if the band it paints is the level it measured. The
+/// thresholds are DESIGN.md's (nominal / caution / peak), and peak-hold is what makes clipping
+/// visible — a bar that follows the moment hides the one audio fault that cannot be fixed later.
+/// </summary>
+public class MeterScaleTests
+{
+    [Fact]
+    public void Bands_peak_hold_and_the_display_scale_follow_the_standard()
+    {
+        // Bands at the documented thresholds.
+        Assert.Equal(MeterBand.Nominal, MeterScale.BandFor(-30f));
+        Assert.Equal(MeterBand.Nominal, MeterScale.BandFor(-12.1f));
+        Assert.Equal(MeterBand.Caution, MeterScale.BandFor(-12f));
+        Assert.Equal(MeterBand.Caution, MeterScale.BandFor(-6f));
+        Assert.Equal(MeterBand.Peak, MeterScale.BandFor(-3f));
+        Assert.Equal(MeterBand.Peak, MeterScale.BandFor(0f));
+
+        // Display fraction: −60 dBFS is empty, 0 dBFS is full, −30 is halfway, and nothing escapes.
+        Assert.Equal(0f, MeterScale.ToFraction(-60f));
+        Assert.Equal(1f, MeterScale.ToFraction(0f));
+        Assert.Equal(0.5f, MeterScale.ToFraction(-30f), 4);
+        Assert.Equal(1f, MeterScale.ToFraction(+12f));
+        Assert.Equal(0f, MeterScale.ToFraction(-120f));
+
+        // Peak hold rises instantly, then decays at the stated rate.
+        Assert.Equal(0.8f, MeterScale.NextPeak(0.2f, 0.8f, 0.05f));
+        Assert.Equal(0.7f, MeterScale.NextPeak(0.8f, 0.1f, 0.1f / 0.35f), 3);   // 0.1 fall at 0.35/s
+        Assert.Equal(0f, MeterScale.NextPeak(0.1f, 0f, 10f));   // never negative
+    }
+}