fix: the overlaid sidebar steps aside for a drag, and the compact view follows the trigger

Two reports, both real, and neither was about the lock itself.

**A drag out of an overlaid sidebar had nowhere to land.** In a window too narrow to show both columns,
`AdwOverlaySplitView` puts the sidebar *on* the plot — that is what `collapsed` means — so the pointer
stays over the list and never over a drop target: a signal could be dragged but not dropped, which reads
as "dragging does not work". `row_drag_sidebar()` now hides the sidebar on `drag-begin` and restores it on
`drag-end`, and only when collapsed: side by side the plot is already visible, and hiding the list the drag
started in would be startling. The reading begins a drag through the *source* with the state forced
collapsed, the way the sidebar probe forces it:

    pane drag: sidebar hidden for the drag=1 restored after it=1

**The compact view did not follow the trigger switch.** The locked line's first branch reads
`tc.enabled` — with trigger mode off it says `stream (trigger off)` and nothing else — and
`on_trigger_toggled()` never re-synced it, so the line kept saying whichever it had been. One call to
`sync_trigger_summary()` beside the switch sync. The dump's two `panes:` lines were already the test and
now pass it: with a session whose trigger is off, the line as found reads `stream (trigger off)` and the
live one reads the settings, because the exit probe turns the trigger on in between. Both reading `stream`
was the defect.

Verified: the two lines above; `panes: the same line as the probe found it: 'mode: stream (trigger off)'`
against the live `'signal: square | condition: window, rising | level: 0.3 .. 0.6 | mode: normal'`;
`make test-ui` 14 files; zero `Gtk-CRITICAL`. Docs: the `pane drag:` reading and the invariants bullet for
the overlay, and the `panes:` pair's second job — the refresh test — written down so the next reader knows
what both lines reading `stream` means.

README.md

@@ -2405,7 +2405,9 @@ now — they are listed so they are not mistaken for anything else:
   on, and neither offers a control that changes the configuration — each heading has an `[edit]` button
   to switch to the full pane, and back. Dragging a signal onto a plot still works while locked — filling
   plots is what the list is for, not configuration — and a locked row can be dragged from anywhere on it,
-  the colour chip included. A **first run** (no session file for that source) comes up
+  the colour chip included. In a window too narrow to show both, where the sidebar is *overlaid on* the
+  plot, the sidebar steps aside for the length of the drag so there is something under the pointer to drop
+  onto. A **first run** (no session file for that source) comes up
   unlocked, because configuring the host is what a first run is for. Each pane's heading also carries its **own switch**, the same control as the
   header bar's button — clicking either moves both — because a pane that is *disabled* while its
   feature is off is a pane whose controls you would otherwise have to leave to turn it back on. The