Added getPos() to SeaweedOutputStream to understand when and how
Hadoop/Parquet queries the output stream position.
Current mystery:
- Files are written correctly (totalBytesWritten=position=chunks)
- But Parquet expects 78 more bytes when reading
- year=2020: wrote 696, expects 774 (missing 78)
- year=2021: wrote 684, expects 762 (missing 78)
The consistent 78-byte discrepancy suggests either:
A) Parquet calculates row group size before finalizing footer
B) FSDataOutputStream tracks position differently than our stream
C) Footer is written with stale/incorrect metadata
D) File size is cached/stale during rename operation
getPos() logging will show if Parquet/Hadoop queries position
and what value is returned vs what was actually written.