feat: locked panes for a returning user, and the trigger is persisted

Two panes, two modes. **Locked** is what a returning user gets: the trigger pane shows one line saying
what it is set to, the signal list shows only the signals that are on, and neither offers a control that
changes the configuration — each heading has an `[edit]` button (the Adwaita `emblem-system-symbolic`,
swapping to `emblem-ok-symbolic` in configuration mode) that flips its pane. A **first run** — no session
file for that source — never reaches `apply_layout()` and comes up in configuration mode, because
configuring the host is what a first run is for. The mode is a mode and not a setting, so it is
deliberately not persisted.

The locked signal list drops the rows of every signal that is off, which is the whole of "show active
signals": the list already filters on nothing, so the lock is one `continue`. With the rows go the acquire
tick, the overflow menu (Information, Calibrate…, Delete, rename) and the `MATH` divider's `+` — and the
colour chip stops being clickable, because choosing a colour is configuration too. Dragging a signal onto
a plot stays: that is which view shows what, not what the scope is set up to do.

**The trigger is persisted, settings and state both** — the settings *even while it is off*, which is the
point: a trigger configured once and switched off is still the trigger you come back to. `LayoutState`
gains fifteen flat `trigger_*` keys; the channel is stored by *name* (every rebuild renumbers the
channels, the model's own rule) and the two enumerations as **the words the panel's dropdowns show**,
because an index ties the file to a declaration order — the lesson `math_op_at()` already carries. The
words are shared with the panel now (`trigger_type_text()` and friends build the dropdown arrays), so the
two cannot drift.

Two ordering traps, both found by running it:

- **The channel name resolves twice over.** `rebuild_trigger_signals()` picks up
  `App::pending_trigger_channel` when the stream arrives *after* the load, and `apply_trigger_layout()`
  resolves it itself when the channels are already there — without the second path the name stayed
  pending past the one rebuild that would have used it, and the trigger silently sat on channel 0
  (`summary='pulse width · sine …'` where the session said `square`).
- **The trigger's state is applied by pressing the control, not by setting a flag**: the toggle is set
  *outside* the `updating_panels` window `apply_layout()` uses, because a toggle set inside it emits
  `toggled` into a handler that returns at once and nothing would happen — no arming, no events button,
  no sensitivity.

Verified, in both modes and both directions:

    panes: trigger=locked signals=locked body_visible=0 summary='window · square · 0.3…0.6 · 20 % pre'
    panes: trigger=config signals=config body_visible=1 summary=''
    signals panel: labels=2 list=1 rows=7 signals=2 math=4 divider=1

The locked trigger pane is 42 px tall against 580 unlocked. A hand-written session round-trips: every
value it named came back in the panel's own controls, `trigger=1/1` (the state arrived and the pane's
switch followed it), and the file the app wrote back carried all fifteen keys with the untouched ones at
their defaults. `[side-tab-probe] … advanced=4` on its documented recipe; `make test-ui` 14 files; zero
`Gtk-CRITICAL` and no missing icon.

Docs: the `panes:` reading with what each field means, the corrected `labels=` note (the panel's frame is
counted above the list now, because the heading sits in a heading row), the invariants for both the
persistence and the lock, the README's deviation note, and the removal of its claim that the trigger has
never been persisted — it is now, and the locked view is built from exactly that.

docs/claude/probe-readings.md

@@ -1139,6 +1139,29 @@ library measures and draws the rect in tests of its own.
   defect this reading exists for, and it was invisible from the frame alone: the file view pinning its
   axis from the ring each frame is what swallowed the gesture.
 
+## `panes:` — the locked and configuration modes
+
+```
+[udpscope]   panes: trigger=locked signals=locked body_visible=0 summary='window · square · 0.25…0.5 · 20 % pre'
+[udpscope]   panes: trigger=config signals=config body_visible=1 summary=''
+```
+
+Both panes have two modes. **Locked** is what a returning user gets — the trigger pane shows the one line
+in `summary`, the signal list shows only the signals that are on, and neither offers a control that
+changes the configuration — and **config** is the full pane, which is what a *first run* gets, because a
+session file that loaded is what "we have been here before" means. Each heading's `[edit]` button flips
+its pane.
+
+- **`summary`** is the locked trigger pane's whole content, so it is read rather than assumed: empty here
+  is a locked pane showing nothing at all. It quotes the shape, the channel, the figures that shape
+  actually reads (a level, two levels, a width plus its comparison, or a timeout) and the pre-trigger
+  share — never a figure the shape ignores.
+- **`body_visible=0`** is the settings being off screen, which is what makes the pane 42 px tall instead
+  of 580 in `panels:`.
+- **The locked signal list drops the rows of the signals that are off**, so `rows` counts what is on:
+  `signals panel: … rows=3 signals=2 divider=1` is two on signals and the `MATH` divider, not four
+  signals. Its `+` goes with them — a `+` on a locked list opens the editor the lock exists to keep shut.
+
 ## `panel switches:` and the trigger panel's default sensitivity
 
 ```
@@ -1330,7 +1353,8 @@ The signals panel's frame read from the window, and in the same line the *shape*
 the heading and the hint the `.ui` gives it, the list the rows go in, how many rows that list holds
 (`rows`), how many of them are streamed signals (`signals`), how many math channels the app has (`math`),
 and `divider`, which is 1 when `rows == signals + 1 + math` and 0 otherwise. `labels=1` is a `.ui` that lost
-one of the two labels, `list=0` one that lost the list -- both of which otherwise show as a panel quietly
+one of the two labels — counted over the panel's subtree *above* the list, because the rows' own name
+and unit labels are the code's and the heading sits in a heading row beside the pane's `[edit]` button — `list=0` one that lost the list -- both of which otherwise show as a panel quietly
 missing a piece -- and **`divider=0` is the merged list that lost its `+`**, the one control that can create
 a math channel. The first line above is a run with no math channels at all: the divider is there anyway,
 because a `+` that only appeared once a channel existed could never create the first one. `panels: …