feat(tui): overhaul latency sparkline scaling, color, and layout

Replace misleading relative-only sparkline with dual-channel design:
bar height uses relative scaling (shows stability and anomalies),
color+brightness uses absolute thresholds (shows fast vs slow).

- Add brightness gradient within color bands (dim→bright as latency
  increases toward the next threshold)
- Pass row background through sparkline rendering so zebra stripes
  and selection highlights carry through ANSI sequences
- Cap sparkline width to 60 (matches maxHistoryLen) and column
  width to 62 to eliminate trailing dead space
- Quiet group sparkline: subtle dots for healthy, bold red for down
- Add braille subpixel canvas (ported from meridian) for future
  multi-row graph use

internal/tui/braille_test.go

@@ -0,0 +1,64 @@
+package tui
+
+import "testing"
+
+func TestBraillePlane_Set(t *testing.T) {
+	p := newBraillePlane(2, 1)
+	if p.wDots != 4 || p.hDots != 4 {
+		t.Fatalf("expected 4x4 dots, got %dx%d", p.wDots, p.hDots)
+	}
+	p.set(0, 0)
+	if !p.dots[0] {
+		t.Error("dot at (0,0) should be set")
+	}
+	p.set(-1, 0) // out of bounds, should not panic
+	p.set(0, 99) // out of bounds, should not panic
+}
+
+func TestBraillePlane_CellMask(t *testing.T) {
+	p := newBraillePlane(1, 1)
+	// Set bottom-left dot
+	p.set(0, 3)
+	mask := p.cellMask(0, 0)
+	if mask != 0x40 {
+		t.Errorf("bottom-left dot should be 0x40, got 0x%02x", mask)
+	}
+	// Set all dots
+	for y := 0; y < 4; y++ {
+		for x := 0; x < 2; x++ {
+			p.set(x, y)
+		}
+	}
+	mask = p.cellMask(0, 0)
+	if mask != 0xFF {
+		t.Errorf("all dots should be 0xFF, got 0x%02x", mask)
+	}
+}
+
+func TestBraillePlane_Line(t *testing.T) {
+	p := newBraillePlane(3, 1)
+	p.line(0, 2, 5, 2) // horizontal line
+	for x := 0; x <= 5; x++ {
+		if !p.dots[2*p.wDots+x] {
+			t.Errorf("dot at (%d, 2) should be set", x)
+		}
+	}
+}
+
+func TestBraillePlane_FillBelow(t *testing.T) {
+	p := newBraillePlane(1, 1)
+	p.set(0, 1) // set dot at row 1
+	p.fillBelow()
+	if !p.dots[1*p.wDots+0] {
+		t.Error("original dot should still be set")
+	}
+	if !p.dots[2*p.wDots+0] {
+		t.Error("row 2 should be filled")
+	}
+	if !p.dots[3*p.wDots+0] {
+		t.Error("row 3 should be filled")
+	}
+	if p.dots[0*p.wDots+0] {
+		t.Error("row 0 above the dot should not be filled")
+	}
+}