ci: keep the e2e job honest on an empty crate

nextest fails a zero-test run, which is the behaviour worth keeping — passing
--no-tests=pass would let a broken filter go green once real tests exist.
Placeholder test instead, same as arr-core.

The first e2e run proved the Transmission service container pulls and starts;
it failed only at the empty test run.

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

crates/arr-e2e/src/lib.rs

@@ -1 +1,13 @@
-//! arr-e2e — see DESIGN.md.
+//! arr-e2e — cross-process integration tests. See DESIGN.md §12.
+//!
+//! Tests here talk to a real Transmission container and to `wiremock` stubs
+//! standing in for Prowlarr and TMDB. Never a live tracker.
+
+#[cfg(test)]
+mod tests {
+    /// Placeholder, same reason as the one in `arr-core`: `cargo nextest`
+    /// fails a zero-test run, and `--no-tests=pass` would let a broken filter
+    /// go green later. Replaced by the real harness in the e2e issue.
+    #[test]
+    fn e2e_crate_builds() {}
+}