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/invariants-gtk-and-chrome.md

@@ -166,9 +166,13 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   heights, so one shared scroller would leave a switch scrolled where the new page never asked to
   be). The reason is the one this trap teaches: five sections competing for one 320 px column left
   every one of them short of room. A *feature's* on/off is no longer a visibility at all -- the
-  trigger's and the cursors' toggles set their pane's **body** insensitive, so the pane stays where it
-  is and the settings you are about to switch back on do not vanish while you look for them. The *body*
-  and not the pane, because each pane's heading carries its own switch (see the next bullet). The
+  **cursors'** toggle sets its pane's **body** insensitive -- the readout has nothing to show with the
+  instruments off -- and the *body* and not the pane, because the pane's heading carries its own switch
+  (see the next bullet). The **trigger's body is deliberately never disabled**: 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. It used to go insensitive with the mode, which left the settings unreachable
+  exactly when a user wanted to change them; `trigger panel sensitive …=1` is the regression guard, and a
+  `0` there means the configuration has been locked out again. The
   trap itself does not go away with the widget: a stack's hidden page measures 0x0 too and is still
   given the leftover width, so **ask about state, not about size** carries over unchanged. Every
   allocation read in the sidebar is a debug capture, and each reads 0 on the page that is *not*