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/sparkline_test.go

@@ -4,10 +4,11 @@ import (
 	"strings"
 	"testing"
 	"time"
+	"unicode/utf8"
 )
 
 func TestLatencySparkline_Empty(t *testing.T) {
-	got := latencySparkline(nil, nil, 10)
+	got := latencySparkline(nil, nil, 10, "")
 	if !strings.Contains(got, "··········") {
 		t.Errorf("empty sparkline should be dots, got %q", got)
 	}
@@ -16,10 +17,13 @@ func TestLatencySparkline_Empty(t *testing.T) {
 func TestLatencySparkline_SingleValue(t *testing.T) {
 	latencies := []time.Duration{100 * time.Millisecond}
 	statuses := []bool{true}
-	got := latencySparkline(latencies, statuses, 5)
+	got := latencySparkline(latencies, statuses, 5, "")
 	if len(got) == 0 {
 		t.Error("sparkline should not be empty")
 	}
+	if !strings.Contains(got, "····") {
+		t.Errorf("single value with width=5 should have 4 dot padding, got %q", got)
+	}
 }
 
 func TestLatencySparkline_WidthTruncation(t *testing.T) {
@@ -29,14 +33,92 @@ func TestLatencySparkline_WidthTruncation(t *testing.T) {
 		latencies[i] = time.Duration(i*50) * time.Millisecond
 		statuses[i] = true
 	}
-	got := latencySparkline(latencies, statuses, 5)
+	got := latencySparkline(latencies, statuses, 5, "")
 	if len(got) == 0 {
 		t.Error("sparkline should not be empty")
 	}
+	if strings.Contains(got, "·") {
+		t.Errorf("20 samples in width=5 should have no padding, got %q", got)
+	}
+}
+
+func TestLatencySparkline_RelativeHeight(t *testing.T) {
+	latencies := []time.Duration{10 * time.Millisecond, 50 * time.Millisecond, 10 * time.Millisecond}
+	statuses := []bool{true, true, true}
+	out := stripANSI(latencySparkline(latencies, statuses, 3, ""))
+	runes := []rune(out)
+	if len(runes) < 3 {
+		t.Fatalf("expected 3 runes, got %d", len(runes))
+	}
+	if runes[0] == runes[1] {
+		t.Errorf("min and max should have different bar heights, got %c %c %c", runes[0], runes[1], runes[2])
+	}
+}
+
+func TestLatencyStyle_BandsProduceDifferentColors(t *testing.T) {
+	sparkSuccess = "#00ff00"
+	sparkWarning = "#ffff00"
+	sparkDanger = "#ff0000"
+	defer func() {
+		sparkSuccess = ""
+		sparkWarning = ""
+		sparkDanger = ""
+	}()
+
+	green := latencyStyle(50, "")
+	yellow := latencyStyle(300, "")
+	red := latencyStyle(800, "")
+
+	gfg := green.GetForeground()
+	yfg := yellow.GetForeground()
+	rfg := red.GetForeground()
+
+	if gfg == yfg || yfg == rfg || gfg == rfg {
+		t.Errorf("bands should produce distinct foreground colors: green=%v yellow=%v red=%v", gfg, yfg, rfg)
+	}
+}
+
+func TestLatencyStyle_BrightnessVariesWithinBand(t *testing.T) {
+	sparkSuccess = "#00ff00"
+	defer func() { sparkSuccess = "" }()
+
+	dim := latencyStyle(10, "")
+	bright := latencyStyle(190, "")
+
+	if dim.GetForeground() == bright.GetForeground() {
+		t.Error("10ms and 190ms should have different brightness within green band")
+	}
+}
+
+func TestLatencySparkline_OutputWidth(t *testing.T) {
+	latencies := []time.Duration{100 * time.Millisecond, 200 * time.Millisecond, 300 * time.Millisecond}
+	statuses := []bool{true, true, true}
+	got := latencySparkline(latencies, statuses, 5, "")
+	count := utf8.RuneCountInString(stripANSI(got))
+	if count != 5 {
+		t.Errorf("expected 5 rune-width output, got %d from %q", count, got)
+	}
+}
+
+func stripANSI(s string) string {
+	var out strings.Builder
+	i := 0
+	for i < len(s) {
+		if s[i] == '\x1b' {
+			for i < len(s) && s[i] != 'm' {
+				i++
+			}
+			i++
+			continue
+		}
+		out.WriteByte(s[i])
+		i++
+	}
+	return out.String()
 }
 
 func TestHeartbeatSparkline_Empty(t *testing.T) {
-	got := heartbeatSparkline(nil, 10)
+	got := heartbeatSparkline(nil, 10, "")
 	if !strings.Contains(got, "··········") {
 		t.Errorf("empty heartbeat should be dots, got %q", got)
 	}
@@ -44,7 +126,7 @@ func TestHeartbeatSparkline_Empty(t *testing.T) {
 
 func TestHeartbeatSparkline_Mixed(t *testing.T) {
 	statuses := []bool{true, false, true, true, false}
-	got := heartbeatSparkline(statuses, 5)
+	got := heartbeatSparkline(statuses, 5, "")
 	if len(got) == 0 {
 		t.Error("heartbeat sparkline should not be empty")
 	}
@@ -52,7 +134,7 @@ func TestHeartbeatSparkline_Mixed(t *testing.T) {
 
 func TestHeartbeatSparkline_PaddedWidth(t *testing.T) {
 	statuses := []bool{true, true}
-	got := heartbeatSparkline(statuses, 5)
+	got := heartbeatSparkline(statuses, 5, "")
 	if !strings.Contains(got, "···") {
 		t.Errorf("should have dot padding for width > data, got %q", got)
 	}