test: the locked signal list keeps its drag, and the reading says so

Asked for: dragging a signal onto a plot must still work in the compact (view) list. It does — the drag
source is on the *row* and was never conditional on the lock — but nothing said so, and "the lock rebuilds
this list and drops three of its controls" is exactly the change that could take the drag with it. Now it
is a reading: **`drag_sources` must be `rows - 1`**, one per signal row, the `MATH` divider not being
draggable.

    signals panel: … rows=3 signals=2 math=0 divider=1 drag_sources=2      (locked)
    signals panel: … rows=5 signals=4 math=0 divider=1 drag_sources=4      (configuration mode)

Measured the direct way first, with a throwaway walk of the list's rows and
`gtk_widget_observe_controllers()`: `locked=1 rows=3 rows_with_a_drag_source=2`, the third row being the
divider. That is where the number in the reading comes from, and the walk is what found the shape of the
answer rather than assuming it.

One consequence worth recording, because it is an improvement rather than a cost: a locked row can be
dragged from *anywhere* on it, the colour chip included. In configuration mode the chip carries a click
gesture that claims the event sequence precisely so a drag cannot start there (it opens the colour
chooser); the locked list does not build that gesture, so the chip is inert and the row's drag source
sees the press.

What the lock does *not* change is which signals are listed: only the ones that are on. A signal nobody
acquires draws nothing on a plot, so a row for it would be a drag that ends in an empty slot.

Verified: both lines above, from sessions differing only in whether one exists; `make test-ui` 14 files;
zero `Gtk-CRITICAL`. Docs: the `drag_sources` field with what a `0` would mean, the invariants' lock
bullet, and the README's note.

docs/claude/probe-readings.md

@@ -1378,7 +1378,11 @@ the two lines together read what the file gave and what the code put in it.
 The signals panel's frame read from the window, and in the same line the *shape* of the one channel list:
 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
+and `divider`, which is 1 when `rows == signals + 1 + math` and 0 otherwise. **`drag_sources` must be
+`rows - 1`** — one per signal row, the `MATH` divider not being draggable. Dragging a signal onto a plot is
+how a plot is filled, so it has to survive both pane modes: the locked list drops the tick, the menu and the
+`+`, and this is the reading that says the drag did not go with them. (`0` is a list that has become a thing
+you can read but not use.) `labels=1` is a `.ui` that lost
 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