feat: live monitor TUI (M2) — 0.21.0

Upgrade `rigdoctor monitor` from a basic redraw to a stdlib curses dashboard
(tui.py): current / session-min / session-max per sensor, grouped by subsystem,
with temperature & utilization color bands (GPU-lost flagged red). q quits,
r resets min/max. Plain full-screen redraw fallback on a non-TTY (--plain forces
it). Pure track()/band() helpers are unit-tested; curses path verified in a pty.

Completes the Monitoring bundle (M2 + M8).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/ROADMAP.md

@@ -22,7 +22,8 @@ Ubuntu + NVIDIA first; `.deb` distribution (see `DECISIONS.md`).
   last readings + a plausible cause.
 
 ## Phase 2 — Live monitor (terminal)
-- [ ] M2 TUI dashboard (current/min/max, grouped, throttle highlighting)
+- [x] M2 TUI dashboard (`rigdoctor monitor`, `tui.py`): curses, current/min/max grouped by
+      subsystem with temp/usage color bands; q quit / r reset; plain-redraw fallback on non-TTY
 - [ ] M8 basic alerting (overheat/throttle/GPU-lost notifications)
 
 ## Phase 3 — Diagnostics breadth