fix: the trigger stays editable while it is off, and the locked pane says "stream"

Two things the lock got wrong, and they are the same mistake seen twice: trigger-off was treated as "the
pane is disabled".

**The configuration was unreachable exactly when it was wanted.** `on_trigger_toggled()` set the trigger
*body* insensitive with the mode — a rule carried over from the five-revealer sidebar, where a closed pane
had nothing to edit. A trigger is configured *before* it is switched on, so switching it off to change it,
or to read what it is set to, is the ordinary way to work. The body is now never disabled; only the
*cursors'* readout still is, which is the pane whose content has nothing to show with the instruments off.

**The locked pane described a trigger that was not running.** With trigger mode off the summary is one
line — `mode: stream (trigger off)` — because the four lines it used to show (signal, condition, level,
mode) describe a trigger that is not there. With the trigger on it is those four plus whatever is not at
its default (`noise band`, `HF reject`, `pre-trigger`, `hold-off`).

The reading changes with it: `trigger panel sensitive …=1` is now a **regression guard** rather than a
state — the trigger's body is always live, so a `0` there means the settings have been locked out again.

Verified in both states, from sessions whose only difference is `trigger_enabled`:

    panes: trigger=locked signals=locked body_visible=0 summary='mode: stream (trigger off)'
    panes: trigger=locked signals=locked body_visible=0 summary='signal: square | condition: window, rising | level: 0.3 .. 0.6 | mode: single | pre-trigger: 20 %'
    trigger panel sensitive before the probe toggled it: 1

The locked trigger pane is 72 px with the trigger off and 192 px with it on. Docs: the invariants' rule
for which body is disabled and why the trigger's is not, the `summary`'s two forms with what each means,
the README's deviation note, and the second `panes:` line renamed to say what it is — the line as the
probe found it, because only a run whose *session* has the trigger off shows the stream form.

README.md

@@ -2412,8 +2412,9 @@ now — they are listed so they are not mistaken for anything else:
   measured), so the pages carry an empty title and the pills are named in code. With the math
   list folded into the signal list under a `MATH` divider whose `+` creates a channel — a derived channel already *is* a channel in
   the model, so the two lists were the same list drawn twice. A tab is one-of-*N*, so no
-  toggle hides a pane any more: trigger-off and cursors-off **disable** their pane and leave
-  it in place, and the measurements' and math panes' toolbar toggles are gone with the panes.
+  toggle hides a pane any more: cursors-off **disables** its readout and leaves it in place, and
+  trigger-off leaves the trigger panel fully editable — a trigger is configured *before* it is
+  switched on, and the locked view says `stream (trigger off)` instead of the settings — and the measurements' and math panes' toolbar toggles are gone with the panes.
 - **A plot holds at most eight signals** (`RowSet::kSlotsPerRow`), because a `Plot`
   cannot give up a trace and the slots are therefore created up front.
 - **One toggle drives both cursor orientations.** The vertical markers and the