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
