test(e2e): let cargo report the daemon binary path

Replaces the current_exe-derived target layout guess with the
compiler-artifact message from the build itself.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/arr-e2e/src/lib.rs

@@ -265,14 +265,18 @@ impl Drop for Daemon {
 /// Build the `arr` binary once per test run and return its path.
 ///
 /// `cargo nextest run -p arr-e2e` compiles only this crate's test binaries,
-/// so the daemon is built here. Cargo's own file locking serialises this
-/// against concurrent test processes, and a warm target directory makes it a
-/// no-op.
+/// so the daemon is built here, and cargo itself reports where the executable
+/// landed — no guessing at the target layout the test runner uses. Cargo's
+/// own file locking serialises this against concurrent test processes, and a
+/// warm target directory makes it a no-op.
 fn daemon_binary() -> &'static PathBuf {
     static BINARY: OnceLock<PathBuf> = OnceLock::new();
     BINARY.get_or_init(|| {
         let mut build = Command::new(env!("CARGO"));
-        build.args(["build", "--quiet", "-p", "arr-daemon", "--bin", "arr"]);
+        build
+            .args(["build", "-p", "arr-daemon", "--bin", "arr"])
+            .args(["--message-format", "json-render-diagnostics"])
+            .stderr(Stdio::inherit());
         // The test process carries the per-crate vars cargo set for *this*
         // crate (CARGO_MANIFEST_DIR, CARGO_PKG_*). Inherited into a nested
         // cargo they poison its fingerprints, so every shell build and every
@@ -285,23 +289,18 @@ fn daemon_binary() -> &'static PathBuf {
                 build.env_remove(key);
             }
         }
-        let status = build.status().expect("run cargo build");
-        assert!(status.success(), "cargo build -p arr-daemon failed");
+        let output = build.output().expect("run cargo build");
+        assert!(output.status.success(), "cargo build -p arr-daemon failed");
 
-        // The test binary runs from target/<profile>/deps/; the daemon lands
-        // one directory up, whatever the target directory and profile are.
-        let mut path = std::env::current_exe().expect("current test binary path");
-        path.pop();
-        if path.ends_with("deps") {
-            path.pop();
-        }
-        let binary = path.join("arr");
-        assert!(
-            binary.is_file(),
-            "expected daemon binary at {}",
-            binary.display()
-        );
-        binary
+        let stdout = String::from_utf8(output.stdout).expect("cargo build output is UTF-8");
+        stdout
+            .lines()
+            .filter_map(|line| serde_json::from_str::<serde_json::Value>(line).ok())
+            .find(|message| {
+                message["reason"] == "compiler-artifact" && message["target"]["name"] == "arr"
+            })
+            .and_then(|message| message["executable"].as_str().map(PathBuf::from))
+            .expect("cargo build reported the arr executable")
     })
 }