Add per-channel independent limit controls

- Separate Lim A / Lim B trackbars (0-200, default 30 each)
- Separate Scale A / Scale B checkboxes (clamp vs scale per channel)
- State.SetLimitA/SetLimitB with independent LimitModeA/LimitModeB
- ConsumeTick applies per-channel limit and mode

HeatMap.cs

@@ -0,0 +1,109 @@
+using System.Drawing.Drawing2D;
+
+namespace Substation;
+
+public class HeatMap : Panel
+{
+    record Tick(byte[] Freq, byte[] Intensity);
+
+    readonly List<Tick> _history = new();
+    readonly int _maxTicks;
+
+    public HeatMap(int maxTicks = 60)
+    {
+        _maxTicks = maxTicks;
+        DoubleBuffered = true;
+        BackColor = Color.FromArgb(20, 20, 22);
+    }
+
+    public void AddTick(byte[] freq, byte[] intensity, bool active)
+    {
+        if (_history.Count >= _maxTicks)
+            _history.RemoveAt(0);
+
+        if (active)
+            _history.Add(new Tick(freq, intensity));
+        else
+            _history.Add(new Tick(Array.Empty<byte>(), Array.Empty<byte>()));
+
+        Invalidate();
+    }
+
+    protected override void OnPaint(PaintEventArgs e)
+    {
+        base.OnPaint(e);
+        var g = e.Graphics;
+        g.SmoothingMode = SmoothingMode.None;
+
+        int w = Width;
+        int h = Height;
+        int colW = Math.Max(1, w / _maxTicks);
+
+        // Draw axis labels
+        using var font = new Font(FontFamily.GenericMonospace, 7);
+        using var textBrush = new SolidBrush(Color.FromArgb(100, 100, 100));
+        g.DrawString("100Hz", font, textBrush, 2, 2);
+        g.DrawString("1Hz", font, textBrush, 2, h - 14);
+
+        int plotX = 36;
+        int plotW = w - plotX - 4;
+        int plotH = h - 4;
+        colW = Math.Max(1, plotW / _maxTicks);
+
+        for (int i = 0; i < _history.Count; i++)
+        {
+            var tick = _history[i];
+            int xPos = plotX + i * colW;
+
+            if (tick.Freq.Length == 0)
+            {
+                // Inactive — dark cell
+                using var dark = new SolidBrush(Color.FromArgb(25, 25, 28));
+                g.FillRectangle(dark, xPos, 2, colW, plotH);
+                continue;
+            }
+
+            // 4 sub-ticks side by side within the column
+            int subW = Math.Max(1, colW / 4);
+            for (int s = 0; s < 4; s++)
+            {
+                int freq = tick.Freq[s];   // 10-240
+                int inten = tick.Intensity[s]; // 0-100
+                if (inten > 100) inten = 0; // 101 sentinel = off
+
+                // Map freq byte (10-240) to Y position (top=high freq, bottom=low freq)
+                float normFreq = (freq - 10f) / (240f - 10f);
+                int y = (int)(2 + plotH * (1f - normFreq));
+                if (y < 2) y = 2;
+                if (y > plotH) y = plotH;
+
+                var color = HeatColor(inten);
+                using var brush = new SolidBrush(color);
+                g.FillRectangle(brush, xPos + s * subW, y, subW, 3);
+            }
+        }
+    }
+
+    static Color HeatColor(int intensity)
+    {
+        // 0=black, 30=blue, 60=green, 80=gold, 100=red
+        if (intensity <= 0) return Color.FromArgb(20, 20, 22);
+        if (intensity < 30)
+        {
+            float t = intensity / 30f;
+            return Color.FromArgb(0, (int)(50 * t), (int)(80 + 80 * t));
+        }
+        if (intensity < 60)
+        {
+            float t = (intensity - 30) / 30f;
+            return Color.FromArgb((int)(60 * t), (int)(130 + 80 * t), (int)(160 - 100 * t));
+        }
+        if (intensity < 80)
+        {
+            float t = (intensity - 60) / 20f;
+            return Color.FromArgb((int)(60 + 180 * t), 210, (int)(60 - 30 * t));
+        }
+        float t2 = (intensity - 80) / 20f;
+        return Color.FromArgb(255, (int)(240 - 140 * t2), (int)(30 - 10 * t2));
+    }
+}