fix: correct mismatched class and struct declarations (#911)

Keep Projections and TableMetadataBuilder::Impl declarations consistent
with their definitions to avoid mismatched-tag diagnostics and potential
link failures under the Microsoft C++ ABI.

Author: zhjwpku

src/iceberg/expression/projections.h

@@ -43,7 +43,7 @@ class ICEBERG_EXPORT ProjectionEvaluator {
   Result<std::shared_ptr<Expression>> Project(const std::shared_ptr<Expression>& expr);
 
  private:
-  friend class Projections;
+  friend struct Projections;
 
   /// \brief Create a ProjectionEvaluator.
   ///

src/iceberg/table_metadata.h

@@ -489,7 +489,7 @@ class ICEBERG_EXPORT TableMetadataBuilder : public ErrorCollector {
   explicit TableMetadataBuilder(const TableMetadata* base);
 
   /// Internal state members
-  struct Impl;
+  class Impl;
   std::unique_ptr<Impl> impl_;
 };