perf(encode): write scalars and scalar arrays without boxing

Assisted-by: GLM 5.3 Flash

cmd/interpres-decode/main_test.go

@@ -567,3 +567,50 @@ type Item struct {
 		}
 	}
 }
+
+func TestRunPlainJSONShapes(t *testing.T) {
+	var stdout, stderr bytes.Buffer
+	in := strings.NewReader("when = 1979-05-27T07:32:00-07:00\nd = 1979-05-27\nt = 07:32:00\nwall = 1979-05-27T07:32:00\n" +
+		"items = [1, \"two\"]\n\n[[tables]]\nx = true\n")
+	code := Run([]string{"-json"}, in, &stdout, &stderr)
+	if code != 0 {
+		t.Fatalf("Run returned %d, want 0; stderr = %q", code, stderr.String())
+	}
+	out := stdout.String()
+	for _, want := range []string{
+		"\"when\": \"1979-05-27T07:32-07:00\"",
+		"\"d\": \"1979-05-27\"",
+		"\"t\": \"07:32\"",
+		"\"wall\": \"1979-05-27T07:32\"",
+		"\"items\": [",
+		"\"x\": true",
+	} {
+		if !strings.Contains(out, want) {
+			t.Errorf("output missing %q:\n%s", want, out)
+		}
+	}
+}
+
+func TestInferStructScalarShapes(t *testing.T) {
+	var stdout, stderr bytes.Buffer
+	in := strings.NewReader("f = 1.5\nb = true\nd = 1979-05-27\nldt = 1979-05-27T07:32:00\nlt = 07:32:00\nnums = [1, 2, 3]\nmixed = [1, \"a\"]\nempty = []\n")
+	code := Run([]string{"-struct"}, in, &stdout, &stderr)
+	if code != 0 {
+		t.Fatalf("Run returned %d, want 0; stderr = %q", code, stderr.String())
+	}
+	out := stdout.String()
+	for _, want := range []string{
+		"F float64",
+		"B bool",
+		"D interpres.LocalDate",
+		"Ldt interpres.LocalDateTime",
+		"Lt interpres.LocalTime",
+		"Nums []int64",
+		"Mixed []any",
+		"Empty []any",
+	} {
+		if !strings.Contains(out, want) {
+			t.Errorf("output missing %q:\n%s", want, out)
+		}
+	}
+}