Boot: zone discovery skips cargo output and the artifacts folder

When the games root is the build's own `target/play`, discovery walked every
file of cargo's output (incremental, deps, build, .fingerprint, examples) and
the artifacts folder, several times at boot: the window sat white and "Not
Responding" for seconds before the map loaded. Those folders never hold
zones, so the walk skips them (retail MW2, MW3 and Black Ops installs have no
folders by these names).

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

crates/asset_transport/src/discover.rs

@@ -250,6 +250,28 @@ fn game_files(root: &Path) -> impl Iterator<Item = Result<PathBuf, String>> {
     files_under(search_roots(root))
 }
 
+/// Folders the game never keeps zones in: cargo's output when the games root is the build's own
+/// `target/play` (tens of thousands of files, walked on every discovery), and our own
+/// artifacts.
+const NOT_GAME_DIRS: [&str; 6] = [
+    "incremental",
+    "deps",
+    "build",
+    ".fingerprint",
+    "examples",
+    "iw4l-artifacts",
+];
+
+fn is_not_game_dir(path: &Path) -> bool {
+    path.file_name()
+        .and_then(|name| name.to_str())
+        .is_some_and(|name| {
+            NOT_GAME_DIRS
+                .iter()
+                .any(|skip| name.eq_ignore_ascii_case(skip))
+        })
+}
+
 fn files_under(roots: Vec<PathBuf>) -> impl Iterator<Item = Result<PathBuf, String>> {
     let mut pending: VecDeque<_> = roots.into_iter().map(Ok).collect();
     let mut visited = HashSet::new();
@@ -259,6 +281,9 @@ fn files_under(roots: Vec<PathBuf>) -> impl Iterator<Item = Result<PathBuf, Stri
                 Ok(path) => path,
                 Err(error) => return Some(Err(error)),
             };
+            if is_not_game_dir(&path) {
+                continue;
+            }
             let canonical = match std::fs::canonicalize(&path) {
                 Ok(path) => path,
                 Err(error) => {