Feat: add integration tests for public API

Co-authored-by: gpt-5.3-codex <no-reply@openai.com>

tests/zwus.rs

@@ -0,0 +1,28 @@
+use zwus::Zwus;
+
+#[test]
+fn roundtrip_string_base_3_6_8() {
+    let text = "secret 🦀 unicode";
+    for base in [3u8, 6, 8] {
+        let encoded = Zwus::encode_string_with_base(text, base);
+        let decoded = Zwus::decode_to_string_with_base(&encoded, base);
+        assert_eq!(decoded, text);
+    }
+}
+
+#[test]
+fn roundtrip_number_array_base_3_6_8() {
+    let nums = [0u32, 72, 101, 108, 108, 111, 0x10FFFF];
+    for base in [3u8, 6, 8] {
+        let encoded = Zwus::encode_number_array_with_base(&nums, base);
+        let decoded = Zwus::decode_to_number_array_with_base(&encoded, base);
+        assert_eq!(decoded, nums);
+    }
+}
+
+#[test]
+fn decode_ignores_non_zwus_characters() {
+    let hidden = Zwus::encode_string_with_base("secret", 6);
+    let mixed = format!("te{hidden}xt");
+    assert_eq!(Zwus::decode_to_string_with_base(&mixed, 6), "secret");
+}