feat: the locked trigger pane carries Arm/Disarm, and one function states both arm buttons

The compact line -- the pane's locked, "what is it set to" mode -- showed the arm state in words and gave
no way to change it, so arming from there meant unfolding the panel first, which is the thing the lock
exists to avoid. The control is built into the pane's *heading*, the row that stays on screen when the
body goes, and it is wired to the same `on_arm_clicked` the body's button uses, so both do one thing.

Its label, sensitivity and visibility are stated by `sync_arm_buttons()`, called from the four places any of
its inputs can move: the click, the mode toggle, the curator's per-tick re-read (a single-shot latch disarms
by itself) and `apply_pane_lock()`. The label comes from the producer's `trigger_armed()` atomic, the other
two from the toolbar toggle that owns `enabled`, so no caller needs the model lock. Visibility is
`enabled && locked`: with trigger mode off the line beside it already reads "stream (trigger off)" and there
is no engine to arm, and with the pane open the body's own button is the one on screen. Never both -- two
arm buttons would be two controls for one state.

Verified with the arm probe, which locks the pane the way a user does (the [edit] click):

    compact arm with the trigger off: visible=0; with the trigger on and the pane locked:
      visible=1 sensitive=1 width=51 label=Disarm
    [udpscope] compact arm probe: locked=1 enabled=1 own=1 effective=1 width=51 panel_width=-1 label=Disarm

`panel_width=-1` is the "not both" assertion: the body's button is not on screen while the compact one is.
The width cost a defect to read. The control is created hidden and shown for the first time inside the
handler, so a same-tick `gtk_widget_get_width()` reads 0 while `gtk_widget_is_visible()` already says 1 --
and the body button's *stale* allocation (270 px, from when the body was still up) reads as a live one,
because `gtk_widget_get_visible()` is the widget's own flag and stays true under a hidden parent. The
reading is therefore taken one frame later, by a step of its own, and the control is compared with
`is_visible()`. Both defects are in the invariants, because the next control read this way will meet them.

0 Gtk-CRITICAL; clean build with the documented four warnings (the help resource re-measured); test-ui
14/14. Docs: the rule and both readings in `invariants-trigger-and-pinning.md`, the probe's entry in
`probe-readings.md`, and the user-facing sentence in `src/help/08-trigger.txt`.

docs/claude/verification.md

@@ -15,7 +15,7 @@ xvfb-run -a env UDPSCOPE_EXIT_AFTER_MS=8000 UDPSCOPE_ENABLE_ALL_MS=1000 \
 
 Two baselines every run is read against. A clean build emits **four** warnings and no more: two
 `-Wmissing-field-initializers` on `MathNode::source` and `-Woverlength-strings` on **both**
-generated GResource `.c` files — the help pages' (**515700** bytes) and the icons', whose blob is
+generated GResource `.c` files — the help pages' (**516044** bytes) and the icons', whose blob is
 **12352** against C99's 4095-byte string-literal limit with the seven action icons and the two app icons
 that remain (a note about a generated file, which GCC itself supports) -- re-measured when
 the tab work changed the set (that icon left, the two tab glyphs arrived, and this file's