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.

docs/claude/probe-readings.md

@@ -1145,6 +1145,7 @@ library measures and draws the rect in tests of its own.
 [udpscope]   pane edits: locked=document-edit-symbolic(ok) config=object-select-symbolic(ok)
 [udpscope]   panes: trigger=locked signals=locked body_visible=0 summary='signal: square | condition: window, falling | level: 0.3 .. 0.6 | mode: single | noise band: 0.2 | pre-trigger: 20 % | hold-off: 1000 ms'
 [udpscope]   panes: trigger=config signals=config body_visible=1 summary=''
+[udpscope]   panes: the same line as the probe found it: 'mode: stream (trigger off)'
 ```
 
 **`pane edits:`** is the two `[edit]` buttons in each mode, by name and verdict, and it exists because of
@@ -1160,6 +1161,14 @@ changes the configuration — and **config** is the full pane, which is what a *
 session file that loaded is what "we have been here before" means. Each heading's `[edit]` button flips
 its pane.
 
+- **With trigger mode off the summary is one line**: `mode: stream (trigger off)`. The four lines below
+  describe a trigger that is not running, so quoting them would describe nothing. The dump carries the line
+  twice — once as the probe found it and once at the end — because only a run whose *session* has the
+  trigger off shows the stream form, and a run whose session has it on shows the settings.
+- **`trigger panel sensitive …=1` is now a guard, not a state.** The trigger's body is never disabled —
+  a trigger is configured before it is switched on — so this reads `1` always, and a `0` means the
+  settings have been made unreachable again. The *cursors'* body is disabled with the cursors, and that
+  is the pane whose readout has nothing to show.
 - **`summary`** is the locked trigger pane's whole content, as `key: value` pairs separated by ` |`. The
   pane draws it as a small definition list — `signal`, `condition`, `level`, `mode` on four lines, and
   then only the figures that are *not* at their defaults (`noise band`, `HF reject`, `pre-trigger`,