feat: decode arrays into [N]T and add MarshalAppend

Assisted-by: GLM 5.3 Flash

encode_test.go

@@ -1998,3 +1998,29 @@ func TestMarshalNumber(t *testing.T) {
 		}
 	})
 }
+
+func TestMarshalAppend(t *testing.T) {
+	buf := []byte("preamble\n")
+	out, err := MarshalAppend(buf, map[string]any{"a": int64(1)})
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := "preamble\na = 1\n"
+	if string(out) != want {
+		t.Errorf("output %q, want %q", out, want)
+	}
+	if &out[0] != &buf[0] {
+		t.Log("append reallocated; capacity differed")
+	}
+	out2, err := MarshalAppend(out, map[string]any{"b": true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(out2) != want+"b = true\n" {
+		t.Errorf("second append %q", out2)
+	}
+	buf = []byte("keep\n")
+	if out3, err := MarshalAppend(buf, Document{}); err == nil {
+		t.Errorf("MarshalAppend with an unencodable value = %q, want an error", out3)
+	}
+}