EMRY-059: TUI chart parity — step-axis, phase bands, checkpoint markers

[vxfemboy]

Bring the TUI braille chart to visual parity with the web dashboard.

The TUI chart is currently index-based (plots the history vector positionally) and ignores phase spans and Checkpoint events. The web chart (EMRY-041/042) is step-based with phase bands and checkpoint markers. This issue closes that gap and is the foundation for the comparison overlay (EMRY-058).

• UiState tracks per-point steps (parallel to history), phase spans, and checkpoint steps — mirroring WebState. Stop dropping Checkpoint events.
• Braille chart becomes step-based: x maps over the visible step window.
• Phase bands: tint chart cell backgrounds by phase over each span (ratatui bg).
• Checkpoint markers: dim vertical marker columns at checkpoint steps.

Milestone: M5 · Depends on EMRY-021 (TUI), EMRY-041/042 (web chart it mirrors) · ~5h