refactor(test): probe permissions instead of checking euid

Replace the `unsafe { libc::geteuid() }` check with a direct probe:
after chmod -r, try read_dir and skip if it still succeeds. This
removes the `unsafe` block and the `libc` dev-dependency while being
more semantically correct — it tests the actual condition we care
about rather than a proxy.

https://claude.ai/code/session_01Ki7wbrgQXWs2GXf8h2hybQ

Author: claude

Cargo.lock

@@ -79,7 +79,6 @@ zero-copy-pads = "0.2.0"
 [dev-dependencies]
 build-fs-tree = "0.8.1"
 command-extra = "1.0.0"
-libc = "0.2"
 maplit = "1.0.2"
 normalize-path = "0.2.1"
 pretty_assertions = "1.4.1"

Cargo.toml

@@ -108,15 +108,16 @@ fn max_depth_0() {
 #[cfg(unix)]
 #[test]
 fn fs_errors() {
-    if unsafe { libc::geteuid() } == 0 {
-        eprintln!("SKIPPED: fs_errors test cannot work as root (permission checks are bypassed)");
-        return;
-    }
-
     let workspace = SampleWorkspace::default();
     fs_permission(workspace.join("empty-dir"), "-r", false);
     fs_permission(workspace.join("nested").join("0"), "-r", false);
 
+    if std::fs::read_dir(workspace.join("empty-dir")).is_ok() {
+        fs_permission(&workspace, "+rwx", true);
+        eprintln!("SKIPPED: permission removal had no effect (likely running as root)");
+        return;
+    }
+
     let Output {
         status,
         stdout,