Fail when the last film stock cannot be reached

The film list bug gave no failure anywhere: the data was right, the
markup compiled, and the list rendered. Two guards now check that the
list can be walked to its end, both by driving input rather than by
reading markup.

tests/film_list_reaches_every_stock.rs runs in CI and needs no display.
It builds the real AppWindow on Slint's testing backend and gives it 28
stocks. It dispatches window events through the same routing a window
uses: popup, Flickables, arbitration. It then checks that the last stock
is on screen, that is, not clipped away:
- after Down past the end, and that Enter chooses it;
- after drags on the list;
- after a run of wheel events with a still pointer;
- after dragging the scrollbar thumb.

Element queries need the Slint compiler's debug tables, which build.rs
emitted only for the `automation` feature. It now emits them for every
debug build too. Release builds, the ones that ship, are unchanged. The
testing backend is a dev-dependency at the same pinned version the
automation feature already uses, so no new crate enters the lockfile.
The test is compiled out of release test runs.

film_reach in tools/manual/scenes.py is the same check on the recording
rig: a real X pointer from xdotool, the release build, and the demo
library. It makes no picture, so it adds nothing to the manual. It runs
with every recording, or alone with `record.sh LIBRARY film_reach`, and
fails the run if the last stock (Ilford HP5 Plus) is out of reach by
the wheel, a drag, the scrollbar or the keys.

Both have to add the popup's position back. The testing backend reports
anything inside a popup relative to the popup, and so does the
automation hook built on it. They take the popup's position from the
Film row and Slint's clamp into the window.

ui/dr-ui/Cargo.toml

@@ -168,3 +168,7 @@ env_logger.workspace = true
 # Standing in a test backend behind `dyn RemoteBackend`, which is an
 # `#[async_trait]` trait — implementing one needs the same attribute.
 async-trait.workspace = true
+# Runs the interface without a display: the film list's reachability test
+# drives the real window's event dispatch on Slint's testing backend. The same
+# pinned release as the `automation` dependency above, so nothing new is built.
+i-slint-backend-testing = { version = "=1.17.1", default-features = false }