feat(writer): Add PositionDeleteFileWriter for position delete files

Add PositionDeleteFileWriter for writing position delete Parquet files
per Iceberg spec. Position deletes use a specific schema with file_path
(field ID 2147483546) and pos (field ID 2147483545) columns.

- Add PositionDeleteWriterConfig with Iceberg spec field IDs
- Implement PositionDeleteFileWriterBuilder with IcebergWriterBuilder trait
- Implement PositionDeleteFileWriter with IcebergWriter trait
- Track referenced_data_file for single-file delete optimization
- Add comprehensive schema validation with detailed error messages
- Add 13 tests covering edge cases, validation, and Parquet field IDs

Author: ethan-tyler

crates/iceberg/src/writer/base_writer/mod.rs

@@ -19,3 +19,4 @@
 
 pub mod data_file_writer;
 pub mod equality_delete_writer;
+pub mod position_delete_writer;