feat: add OffsetDateTime, nesting limits and uniform Marshaler dispatch

Assisted-by: DeepSeek V4.1 Flash

cmd/interpres-decode/main.go

@@ -285,8 +285,11 @@ func typeMatches(typ string, v any) bool {
 		_, ok := v.(bool)
 		return ok
 	case "datetime":
-		_, ok := v.(time.Time)
-		return ok
+		switch v.(type) {
+		case time.Time, interpres.OffsetDateTime:
+			return true
+		}
+		return false
 	case "datetime-local":
 		_, ok := v.(interpres.LocalDateTime)
 		return ok
@@ -346,6 +349,8 @@ func tag(v any) (any, error) {
 		return tagged("float", formatFloat(x)), nil
 	case time.Time:
 		return tagged("datetime", x.Format(time.RFC3339Nano)), nil
+	case interpres.OffsetDateTime:
+		return tagged("datetime", x.Format(time.RFC3339Nano)), nil
 	case interpres.LocalDateTime:
 		return tagged("datetime-local", x.Format("2006-01-02T15:04:05.999999999")), nil
 	case interpres.LocalDate:

cmd/interpres-decode/main_test.go

@@ -373,7 +373,7 @@ func TestRunEncoderRejectsBadInput(t *testing.T) {
 		{"top-level-array", `[{"type": "integer", "value": "1"}]`, "must be a JSON object"},
 		{"untagged-scalar", `{"x": 1}`, "unsupported JSON value"},
 		{"literal-mismatch", `{"x": {"type": "integer", "value": "1.5"}}`, "parsed as float64"},
-		{"offset-for-local", `{"x": {"type": "datetime-local", "value": "1979-05-27T07:32:00Z"}}`, "parsed as time.Time"},
+		{"offset-for-local", `{"x": {"type": "datetime-local", "value": "1979-05-27T07:32:00Z"}}`, "parsed as interpres.OffsetDateTime"},
 		{"bad-literal", `{"x": {"type": "date-local", "value": "nope"}}`, "date-local"},
 		{"smuggled-statement", `{"x": {"type": "integer", "value": "1\nx = 2"}}`, "not a single bare value"},
 	}