ui: Arm is disabled with trigger mode off, and the redundant Go-live button is gone

**`Arm` is now disabled while trigger mode is off**, and labelled `Arm` again: it is the one control in
that pane that is *runtime* rather than setting, so with no engine running there is nothing to arm.
It is also the only thing in the body that stays disabled — the settings themselves stay editable, which
is the point of the previous commit. `arm sensitive with the trigger off=0` is the reading.

**`Go live (trigger off)` is deleted.** Its handler was `trigger_toggle → FALSE`, i.e. exactly the
toolbar's trigger switch and the pane's own switch, which is what it was for: "the way out of trigger
mode, from inside the panel that froze the view: it is the toolbar's trigger switch, so there is exactly
one piece of state." With that switch now in the pane's own heading, the button was the same control
drawn a third time. The status line that told users about it says "or switch the trigger off to stream"
instead. The events pane's own `events_live` button is a different button on a different path — the file
view restore — and is untouched.

**And one silent regression this work uncovered.** The `trigger panel:` frame reading had been walking the
*wrapper* box the app puts around the `.ui`'s panel (heading row + locked summary + panel) ever since the
heading moved out of the file in the lock commit, so it reported `boxes=2 … search_inside=0/4` against a
panel that was entirely correct. It reads `trigger_body` — the file's own panel — again, and now says so in
its comment, because a reading that can silently start describing the wrong widget is the failure it exists
to catch.

Verified: `trigger panel: boxes=15 controls_inside=15/15 search_widgets=4/4 search_inside=4/4` (the
documented shape, restored); `arm sensitive with the trigger off=0`; `trigger panel sensitive …=1`;
`panes:`/`panel switches:`/`pane edits:` unchanged; `make test-ui` 14 files; zero `Gtk-CRITICAL`.

docs/claude/probe-readings.md

@@ -1169,6 +1169,9 @@ its pane.
   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.
+- **`arm sensitive with the trigger off=0`** is the one exception inside that body: `Arm` is a *runtime*
+  control, so with trigger mode off there is no engine to arm and it is disabled (and labelled `Arm`
+  again). `1` here is an Arm button that does nothing when clicked.
 - **`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`,
@@ -1348,7 +1351,11 @@ through one row type, for the reason in `invariants-gtk-and-chrome.md`. The form
 [udpscope]   trigger panel: boxes=15 controls_inside=15/15 search_widgets=4/4 search_inside=4/4
 ```
 
-The trigger panel's frame, read from the window: `boxes` is the row boxes the `.ui` holds, `controls_inside`
+The trigger panel's frame, read from the window: **the `.ui`'s own panel** (`trigger_body`), not the box
+this app wraps it in — the heading row and the locked summary are the code's arrangement, and reading the
+*wrapper* made this line report `boxes=2 … search_inside=0/4` the moment the heading moved out of the file.
+That is the failure this line is for, so it is worth knowing it can happen to itself. `boxes` is the row
+boxes the `.ui` holds, `controls_inside`
 counts the fifteen controls the code built and then appended into those rows -- the last two being the
 noise band and the HF reject, whose rows arrived with the noise-rejection work, `search_widgets` is the four
 search buttons having resolved by id, and `search_inside` is those four being inside the block the expander