test(envd): add 136 unit tests across 12 modules

Cover all pure-function modules with inline #[cfg(test)] blocks:
crypto (NIST/RFC 4231 known-answer vectors), auth (SecureToken ops,
signature generation/validation), conntracker (snapshot lifecycle),
execcontext, util (AtomicMax concurrent correctness), http/encoding
(RFC 7231 negotiation), port/conn (/proc/net/tcp parsing),
rpc/entry (format_permissions), and permissions/path (tilde expansion,
ensure_dirs). Add tempfile dev-dep for filesystem tests. Update
Makefile test target to include cargo test.

envd-rs/src/permissions/path.rs

@@ -70,3 +70,115 @@ pub fn ensure_dirs(path: &str, uid: Uid, gid: Gid) -> Result<(), String> {
 
     Ok(())
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    // expand_tilde
+
+    #[test]
+    fn tilde_empty_passthrough() {
+        assert_eq!(expand_tilde("", "/home/u").unwrap(), "");
+    }
+
+    #[test]
+    fn tilde_no_tilde_passthrough() {
+        assert_eq!(expand_tilde("/absolute", "/home/u").unwrap(), "/absolute");
+    }
+
+    #[test]
+    fn tilde_bare() {
+        assert_eq!(expand_tilde("~", "/home/user").unwrap(), "/home/user");
+    }
+
+    #[test]
+    fn tilde_slash_path() {
+        assert_eq!(expand_tilde("~/docs", "/home/user").unwrap(), "/home/user/docs");
+    }
+
+    #[test]
+    fn tilde_nested() {
+        assert_eq!(expand_tilde("~/a/b/c", "/h").unwrap(), "/h/a/b/c");
+    }
+
+    #[test]
+    fn tilde_other_user_errors() {
+        assert!(expand_tilde("~bob/foo", "/home/user").is_err());
+    }
+
+    #[test]
+    fn tilde_relative_no_tilde() {
+        assert_eq!(expand_tilde("relative/path", "/home/u").unwrap(), "relative/path");
+    }
+
+    // expand_and_resolve
+
+    #[test]
+    fn resolve_absolute_passthrough() {
+        assert_eq!(expand_and_resolve("/abs/path", "/home", None).unwrap(), "/abs/path");
+    }
+
+    #[test]
+    fn resolve_empty_uses_default() {
+        assert_eq!(expand_and_resolve("", "/home", Some("/default")).unwrap(), "/default");
+    }
+
+    #[test]
+    fn resolve_empty_no_default_falls_back_to_home() {
+        // Empty path with no default → joins "" with home_dir → returns home_dir
+        let result = expand_and_resolve("", "/home", None).unwrap();
+        assert_eq!(result, "/home");
+    }
+
+    #[test]
+    fn resolve_tilde_expands() {
+        assert_eq!(expand_and_resolve("~/dir", "/home/u", None).unwrap(), "/home/u/dir");
+    }
+
+    #[test]
+    fn resolve_relative_joins_home() {
+        let result = expand_and_resolve("subdir", "/tmp", None).unwrap();
+        // Relative path joined with home and canonicalized (or raw join on missing)
+        assert!(result.starts_with("/tmp"));
+        assert!(result.contains("subdir"));
+    }
+
+    #[test]
+    fn resolve_tilde_other_user_errors() {
+        assert!(expand_and_resolve("~bob", "/home/u", None).is_err());
+    }
+
+    // ensure_dirs
+
+    #[test]
+    fn ensure_dirs_creates_nested() {
+        let tmp = tempfile::TempDir::new().unwrap();
+        let path = tmp.path().join("a/b/c");
+        let uid = nix::unistd::getuid();
+        let gid = nix::unistd::getgid();
+        ensure_dirs(path.to_str().unwrap(), uid, gid).unwrap();
+        assert!(path.is_dir());
+    }
+
+    #[test]
+    fn ensure_dirs_existing_is_ok() {
+        let tmp = tempfile::TempDir::new().unwrap();
+        let uid = nix::unistd::getuid();
+        let gid = nix::unistd::getgid();
+        ensure_dirs(tmp.path().to_str().unwrap(), uid, gid).unwrap();
+    }
+
+    #[test]
+    fn ensure_dirs_file_in_path_errors() {
+        let tmp = tempfile::TempDir::new().unwrap();
+        let file_path = tmp.path().join("afile");
+        std::fs::write(&file_path, "").unwrap();
+        let nested = file_path.join("subdir");
+        let uid = nix::unistd::getuid();
+        let gid = nix::unistd::getgid();
+        let result = ensure_dirs(nested.to_str().unwrap(), uid, gid);
+        assert!(result.is_err());
+        assert!(result.unwrap_err().contains("path is a file"));
+    }
+}