fix: result type of DayTransform should be date

Result type of Day transform should be `int` per the spec, but in
the Java impl, it is `DateType`, this discrepancy has been discussed
on devlist, see [1]. iceberg-iceberg and iceberg-rust all conform to
the java impl, so make iceberg-cpp do the same.

The added comments are borrowed from iceberg-python.

[1] https://lists.apache.org/thread/2gq7b54nvc9q6f1j08l9lnzgm5onkmx5

Author: zhjwpku

src/iceberg/test/transform_test.cc

@@ -134,7 +134,7 @@ TEST(TransformResultTypeTest, PositiveCases) {
        .expected_result_type = iceberg::int32()},
       {.str = "day",
        .source_type = iceberg::timestamp(),
-       .expected_result_type = iceberg::int32()},
+       .expected_result_type = iceberg::date()},
       {.str = "hour",
        .source_type = iceberg::timestamp(),
        .expected_result_type = iceberg::int32()},

src/iceberg/transform.h

@@ -202,6 +202,11 @@ class ICEBERG_EXPORT TransformFunction {
   /// \brief Get the source type of transform function
   const std::shared_ptr<Type>& source_type() const;
   /// \brief Get the result type of transform function
+  ///
+  /// Note: This method defines both the physical and display representation of the
+  /// partition field. The physical representation must conform to the Iceberg spec. The
+  /// display representation can deviate from the spec, such as by transforming the value
+  /// into a more human-readable format.
   virtual std::shared_ptr<Type> ResultType() const = 0;
 
   friend bool operator==(const TransformFunction& lhs, const TransformFunction& rhs) {

src/iceberg/transform_function.cc

@@ -193,7 +193,7 @@ Result<Literal> DayTransform::Transform(const Literal& literal) {
   return TemporalUtils::ExtractDay(literal);
 }
 
-std::shared_ptr<Type> DayTransform::ResultType() const { return int32(); }
+std::shared_ptr<Type> DayTransform::ResultType() const { return date(); }
 
 Result<std::unique_ptr<TransformFunction>> DayTransform::Make(
     std::shared_ptr<Type> const& source_type) {

src/iceberg/transform_function.h

@@ -141,7 +141,11 @@ class ICEBERG_EXPORT DayTransform : public TransformFunction {
   /// \brief Extract a date or timestamp day, as days from 1970-01-01.
   Result<Literal> Transform(const Literal& literal) override;
 
-  /// \brief Returns INT32 as the output type.
+  /// \brief Return the result type of a day transform.
+  ///
+  /// Note: The physical representation conforms to the Iceberg spec as DateType is
+  /// internally converted to int. The DateType returned here provides a more
+  /// human-readable way to display the partition field.
   std::shared_ptr<Type> ResultType() const override;
 
   /// \brief Create a DayTransform.