feat(tui): add Diagnostics panel for stored evidence

Toggle a Diagnostics panel with i, built on the Activity readout. It swaps
in for the Drive pane and shows the decoded critical_warning, power-on
hours, power cycles, unsafe shutdowns, media errors and available spare
with their change over the selection, and per-hour (Day) or per-day
(History) temperature min/avg/max, active/idle/powered-off/unknown split
and coverage. History bars draw reads beside writes while it is open, and a
set critical_warning shows as a badge in the endurance outlook without
affecting the projection. Counter deltas are withheld across a controller
segment change.

README.md

@@ -275,6 +275,13 @@ UTC-only summaries cannot establish exact local-day totals.
   `PgUp` / `PgDn` jump a week in Day and History (the live window is three
   hours); the mouse wheel moves the selection too. `w`
   switches read/write volume in every view.
+- `i` swaps the Drive pane for a Diagnostics panel built from the stored
+  evidence: the decoded `critical_warning`, power-on hours, power cycles,
+  unsafe shutdowns, media errors and available spare with their change over the
+  selection, and per-hour (Day) or per-day (History) temperature min/avg/max,
+  active / idle / powered-off / unknown split and coverage. History bars then
+  draw reads beside writes. A set `critical_warning` also shows as a badge in
+  the endurance outlook; it never changes the projection.
 - `[` / `]` browse dates, `g` enters a date, and `t` returns to today/live.
 - `Tab` / `Shift+Tab` move focus between panels (clicking a panel focuses it);
   `z` expands the focused panel, and `z` or `Esc` restores it. Monitoring status and controls remain visible.