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/view_history.go

@@ -47,6 +47,8 @@ func computeHistoryStats(changes []models.StateChange) historyStats {
 	return s
 }
 
+var stateChangeChars = []rune{'▁', '▂', '▃', '▄', '▅', '▆', '▇', '█'}
+
 func stateChangeSparkline(changes []models.StateChange, width int) string {
 	if len(changes) < 2 || width < 4 {
 		return ""
@@ -91,7 +93,7 @@ func stateChangeSparkline(changes []models.StateChange, width int) string {
 		if idx > 7 {
 			idx = 7
 		}
-		ch := string(sparkChars[idx])
+		ch := string(stateChangeChars[idx])
 		switch {
 		case v >= 3:
 			sb.WriteString(dangerStyle.Render(ch))