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/invariants-session-and-export.md

@@ -18,7 +18,11 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   a control that changes the configuration. A *first run* — no session file for that source — never
   reaches `apply_layout()` and stays in configuration mode, which is what a first run is for. The mode
   is a mode and not a setting: it is deliberately not persisted, so a scope reopened tomorrow starts
-  where a returning user expects to start.
+  where a returning user expects to start. **Dragging a row onto a plot is not configuration and survives
+  the lock**: filling plots is what the list is *for*, so a locked row keeps its `GtkDragSource` — from
+  anywhere on it, the colour chip included, which in configuration mode has a click gesture that claims
+  the sequence precisely so a drag cannot start there. `drag_sources` on the `signals panel:` line is the
+  reading, and it must be `rows - 1`.
 - **The session is the layout.** `LayoutState` persists what makes a reopened scope look like the
   one that was closed: the window, the rows and their slot signals, the calibration, **the enabled
   signals**, **the chosen trace colours** (`[[color]]`, `name` + `rgb = "#RRGGBB"` — only the