Tests: make the Windows-only failures pass (paths with /, MCP sockets vs child processes)

On Windows 11, `cargo test --workspace` on main fails 11 tests:

- tests_export::same_folder_as_original_exports_next_to_each_photo built its
  expected paths with Path::join (`\`), but export joins the subfolder and
  file name with `/` (join_path) on every platform. Expect what export writes.
- tests_folder_drop (8 tests) looked rows up as
  `source:libfolder:C:\...`, while the Folders section names them with `/`
  (`source:libfolder:C:/...`), so no row was found. Look them up the same way.
- mcp backend::tests::{valid_replies_and_errors_are_preserved,
  malformed_replies_are_rejected_without_resending} failed only when the whole
  crate ran: the shutdown and one-shot-process tests start a copy of the test
  binary, the child can inherit a socket a fake server just accepted, and when
  the child exits Windows closes its copy abortively, resetting the test's
  connection. A test-only lock keeps socket tests and child-process tests from
  overlapping (passes 6 of 6 whole-crate runs, failed 4 of 4 before).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/tests_export.rs

@@ -991,7 +991,8 @@ fn same_folder_as_original_exports_next_to_each_photo() {
     let exists = |p: &str| std::path::Path::new(p).exists();
     let files = crate::export::export_batch(&mut s, &ids, &o, &to, &mut crate::export::write_file, &exists).unwrap();
     let written: Vec<&str> = files.iter().map(|f| f["path"].as_str().unwrap()).collect();
-    let mut want = vec![a.join("Exports/IMG_1.jpg").to_string_lossy().to_string(), b.join("Exports/IMG_1.jpg").to_string_lossy().to_string()];
+    // `export` joins the subfolder and name with `/` (`join_path`), on Windows too.
+    let mut want = vec![format!("{}/Exports/IMG_1.jpg", a.to_string_lossy()), format!("{}/Exports/IMG_1.jpg", b.to_string_lossy())];
     let mut got: Vec<String> = written.iter().map(|p| p.to_string()).collect();
     want.sort();
     got.sort();

crates/mcp/src/backend.rs

@@ -193,6 +193,7 @@ mod tests {
     use std::sync::atomic::{AtomicUsize, Ordering};
 
     fn with_reply(reply: &[u8], check: impl FnOnce(&mut Remote, Result<Value, String>)) {
+        let _serial = crate::serial_sockets_and_children();
         let listener = TcpListener::bind("127.0.0.1:0").unwrap();
         let addr = listener.local_addr().unwrap().to_string();
         let reply = reply.to_vec();
@@ -307,6 +308,7 @@ mod tests {
 
     #[test]
     fn consecutive_replies_match_their_requests_on_one_connection() {
+        let _serial = crate::serial_sockets_and_children();
         let listener = TcpListener::bind("127.0.0.1:0").unwrap();
         let addr = listener.local_addr().unwrap().to_string();
         let worker = std::thread::spawn(move || {
@@ -331,6 +333,7 @@ mod tests {
 
     #[test]
     fn lost_reply_is_not_resent() {
+        let _serial = crate::serial_sockets_and_children();
         let listener = TcpListener::bind("127.0.0.1:0").unwrap();
         let addr = listener.local_addr().unwrap().to_string();
         let seen = Arc::new(AtomicUsize::new(0));
@@ -353,6 +356,7 @@ mod tests {
 
     #[test]
     fn connection_closed_by_app_reconnects_before_sending() {
+        let _serial = crate::serial_sockets_and_children();
         let listener = TcpListener::bind("127.0.0.1:0").unwrap();
         let addr = listener.local_addr().unwrap().to_string();
         let seen = Arc::new(AtomicUsize::new(0));

crates/mcp/src/lib.rs

@@ -33,3 +33,14 @@ pub const DEFAULT_ADDR: &str = "127.0.0.1:7980";
 
 #[cfg(test)]
 mod tests;
+
+/// Held by tests that start a child process (a copy of the test binary) and by tests that run a
+/// loopback socket server, so the two never overlap. On Windows the child can inherit a socket the
+/// server just accepted; when the child exits, Windows closes its copy abortively and the test's
+/// connection is reset ("An existing connection was forcibly closed"), which failed the
+/// `backend` reply tests whenever they ran alongside a child-process test.
+#[cfg(test)]
+pub(crate) fn serial_sockets_and_children() -> std::sync::MutexGuard<'static, ()> {
+    static LOCK: std::sync::Mutex<()> = std::sync::Mutex::new(());
+    LOCK.lock().unwrap_or_else(std::sync::PoisonError::into_inner)
+}

crates/mcp/src/progress_tests.rs

@@ -100,6 +100,7 @@ fn progress_flush_failure_does_not_wait_for_input_eof() {
 }
 
 fn run_shutdown_case(mode: &str) {
+    let _serial = crate::serial_sockets_and_children();
     let mut child = Command::new(std::env::current_exe().unwrap())
         .args(["--exact", "server::progress::shutdown_tests::output_failure_child", "--ignored", "--nocapture"])
         .env("LIGHTCRAFT_MCP_SHUTDOWN_CASE", mode)

crates/mcp/src/tests.rs

@@ -225,6 +225,7 @@ fn a_one_shot_process_never_ends_during_a_photos_import() {
         drop(h); // what lightcraft-cli does before it exits
         return;
     }
+    let _serial = crate::serial_sockets_and_children();
     for mode in ["default", "nowait"] {
         let dir = std::env::temp_dir().join(format!("lc-mcp-photos-child-{mode}-{}", std::process::id()));
         let _ = std::fs::remove_dir_all(&dir);

crates/ui-egui/src/tests_folder_drop.rs

@@ -50,8 +50,13 @@ fn widget(h: &Headless, id: &str) -> Option<Rect> {
     h.app.widgets.iter().find(|(w, _)| w == id).map(|(_, r)| *r)
 }
 
+/// How the Folders section names `dir` in widget ids: with `/` separators, on Windows too.
+fn folder_key(dir: &Path) -> String {
+    dir.to_string_lossy().replace('\\', "/")
+}
+
 fn folder_row(h: &Headless, dir: &Path) -> Option<Rect> {
-    widget(h, &format!("source:libfolder:{}", dir.to_string_lossy()))
+    widget(h, &format!("source:libfolder:{}", folder_key(dir)))
 }
 
 fn id_of(h: &Headless, file: &str) -> u64 {
@@ -180,7 +185,7 @@ fn photos_already_in_the_folder_stay_put() {
     let (root, trip, home) = folders("here");
     let mut h = app(&[(trip.join("beach.png"), false), (home.join("garden.png"), false)]);
     // open the folder holding both
-    let toggle = widget(&h, &format!("libraryFolderToggle:{}", root.to_string_lossy())).unwrap();
+    let toggle = widget(&h, &format!("libraryFolderToggle:{}", folder_key(&root))).unwrap();
     h.app.synthetic.push(egui::Event::PointerMoved(toggle.center()));
     h.app.synthetic.push(egui::Event::PointerButton {
         pos: toggle.center(),
@@ -284,7 +289,7 @@ fn cancelling_the_question_moves_nothing() {
     let target = if folder_row(&h, &home).is_some() {
         target
     } else {
-        click(&mut h, &format!("libraryFolderToggle:{}", root.to_string_lossy()));
+        click(&mut h, &format!("libraryFolderToggle:{}", folder_key(&root)));
         folder_row(&h, &home).unwrap()
     };
     let undo = h.app.session.undo.len();
@@ -302,7 +307,7 @@ fn dont_ask_again_moves_the_next_drop_at_once() {
     let (root, trip, home) = folders("dont-ask");
     std::fs::copy(trip.join("beach.png"), trip.join("dunes.png")).unwrap();
     let mut h = app(&[(trip.join("beach.png"), false), (trip.join("dunes.png"), false), (home.join("garden.png"), false)]);
-    click(&mut h, &format!("libraryFolderToggle:{}", root.to_string_lossy()));
+    click(&mut h, &format!("libraryFolderToggle:{}", folder_key(&root)));
     let target = folder_row(&h, &home).unwrap();
     let beach = id_of(&h, "beach.png");
     h.app.session.selection = lightcraft_engine::Selection::single(lightcraft_catalog::PhotoId(beach));
@@ -327,7 +332,7 @@ fn dont_ask_again_moves_the_next_drop_at_once() {
 fn a_question_left_open_while_another_library_opens_moves_nothing() {
     let (root, trip, home) = folders("other-library");
     let mut h = app(&[(trip.join("beach.png"), false), (home.join("garden.png"), false)]);
-    click(&mut h, &format!("libraryFolderToggle:{}", root.to_string_lossy()));
+    click(&mut h, &format!("libraryFolderToggle:{}", folder_key(&root)));
     let beach = id_of(&h, "beach.png");
     let target = folder_row(&h, &home).unwrap();
     drop_beach(&mut h, beach, target);