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

Assisted-by: GLM 5.3 Flash

decode_test.go

@@ -1273,3 +1273,46 @@ func TestSourceLineEdgePositions(t *testing.T) {
 		t.Errorf("SourceLine(offset past end) =\n%q\nwant:\n%q", got, want)
 	}
 }
+
+func TestDecodeFixedArray(t *testing.T) {
+	t.Run("value array", func(t *testing.T) {
+		var cfg struct {
+			Ports [2]int      `toml:"ports"`
+			Label [2]string   `toml:"label"`
+			Grid  [2][2]int64 `toml:"grid"`
+		}
+		in := []byte("ports = [8080, 9090]\nlabel = [\"a\", \"b\"]\ngrid = [[1, 2], [3, 4]]\n")
+		if err := Unmarshal(in, &cfg); err != nil {
+			t.Fatal(err)
+		}
+		if cfg.Ports != [2]int{8080, 9090} || cfg.Label != [2]string{"a", "b"} || cfg.Grid != [2][2]int64{{1, 2}, {3, 4}} {
+			t.Errorf("decoded %+v", cfg)
+		}
+	})
+	t.Run("array of tables", func(t *testing.T) {
+		type Item struct {
+			Name string `toml:"name"`
+			Qty  int    `toml:"qty"`
+		}
+		var cfg struct {
+			Items [2]Item `toml:"items"`
+		}
+		in := []byte("[[items]]\nname = \"a\"\nqty = 1\n[[items]]\nname = \"b\"\nqty = 2\n")
+		if err := Unmarshal(in, &cfg); err != nil {
+			t.Fatal(err)
+		}
+		if cfg.Items != [2]Item{{"a", 1}, {"b", 2}} {
+			t.Errorf("decoded %+v", cfg)
+		}
+	})
+	t.Run("a length mismatch is an error", func(t *testing.T) {
+		var cfg struct {
+			Ports [3]int `toml:"ports"`
+		}
+		err := Unmarshal([]byte("ports = [8080, 9090]\n"), &cfg)
+		want := "ports: interpres: cannot assign 2 elements to [3]int"
+		if err == nil || err.Error() != want {
+			t.Errorf("err = %v, want %q", err, want)
+		}
+	})
+}