Keep the runner's borrow alive as long as the connection it reads

The first compile this branch had. One borrow error, in the four-thread
contention test: the `Runner` was the block's tail expression, and a
tail's temporaries are dropped after the block's locals, so it outlived
the `conn` it borrowed. Bound to a local, with the ordering rule written
down beside it -- it is exactly the shape someone tidies back.

Everything else stood: clippy clean at -D warnings, and all 18 runner
tests pass, including the four-thread four-connection claim and the
`UPDATE ... RETURNING` rewrite the author flagged as the riskiest line
in the diff.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

core/dr-catalog/src/runner.rs

@@ -853,11 +853,16 @@ mod tests {
             let seen = seen.clone();
             threads.push(std::thread::spawn(move || {
                 let conn = open(&path);
-                Runner::new(&conn)
+                // Bound to a local rather than left as the block's tail: the
+                // `Runner` borrows `conn`, and a tail expression's temporaries
+                // are dropped *after* the block's locals, so the borrow would
+                // outlive what it borrows.
+                let completed = Runner::new(&conn)
                     .with(recording(vec![JobKind::Thumbnail], seen))
                     .drain_all(0)
                     .unwrap()
-                    .completed
+                    .completed;
+                completed
             }));
         }