fix: the [edit] buttons' icons, and the locked pane says four lines not one

Two things the lock shipped wrong.

**The "done" icon did not exist.** `emblem-ok-symbolic` is a breeze icon; Adwaita — which is what this
app runs under — does not have it, so the button that leaves configuration mode drew a missing-image
placeholder, i.e. nothing. Both names are now Adwaita's own: `document-edit-symbolic` (the pencil: edit)
and `object-select-symbolic` (the check: done). This is exactly the silent failure `toolbar icons:`
exists for, so the two `[edit]` buttons get the same treatment: `pane edits: locked=…(ok) config=…(ok)`
is read by clicking into configuration mode and back, so one line carries both names and
`gtk_icon_theme_has_icon()`'s verdict for each. The invariants now carry the rule, including the trap
that a name found under `breeze` is not an answer.

**The locked pane was too compact.** One line could not hold what a user opens the pane for, so it is now
a small definition list: `signal`, `condition`, `level`, `mode` on four lines, and after that only the
figures that are *not* at their defaults — `noise band`, `HF reject`, `pre-trigger`, `hold-off` — because
a line reading "band 0" is a line that says nothing. The channel is resolved from the acquired channels
and the condition quotes the figures its own shape reads (a width and its comparison, two levels, or a
timeout), never one it ignores. The pane is ~192 px tall locked against 580 in configuration mode.

One self-inflicted critical on the way: the probe read the summary with `gtk_label_get_text()`, and the
container is a grid now — `Gtk-CRITICAL: assertion 'GTK_IS_LABEL (self)' failed`. The text comes from the
sync that writes it instead.

Verified: `pane edits: locked=document-edit-symbolic(ok) config=object-select-symbolic(ok)`;
`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'`;
`make test-ui` 14 files; zero `Gtk-CRITICAL` and no missing icon.

docs/claude/invariants-gtk-and-chrome.md

@@ -184,6 +184,13 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   **tooltip and accessible name** from the page's name, then hides the empty label that would otherwise
   leave the icon off-centre. An icon-only pill is still a named control for a screen reader, and
   `switcher: … pills=2 labels=0 names=2` is the reading for all three facts at once.
+- **A stock icon name has to be one *Adwaita* has, and a blank button is the failure mode.** The two
+  panes' `[edit]` buttons are `document-edit-symbolic` (the pencil: edit) and `object-select-symbolic`
+  (the check: done). The done one was `emblem-ok-symbolic`, which breeze ships and **Adwaita does not** —
+  so the button that leaves configuration mode was blank, which no other reading could see: the button
+  worked, the layout was unchanged, and only `pane edits:` (the two names and
+  `gtk_icon_theme_has_icon()`'s verdict, clicked into each mode) would say so. Check a name against
+  `/usr/share/icons/Adwaita` before using it; a name found under `breeze` is not an answer.
 - **A pane's heading switch and the header bar's button are one control in two places.** Each pane's
   heading (`build_trigger_panel()` builds the trigger's, `cursors-panel.ui` has the cursors' in its
   `head`) carries a `GtkSwitch` that runs the *same action* the toolbar button runs, and