Expose ColumnStatistics from MosaicWriter for all language bindings (#16)

After close(), writer stats are available without re-reading the file.
This is useful for Paimon to obtain min/max statistics immediately
after writing for indexing and metadata purposes.

Changes across all layers:
- Rust core: add num_row_groups() and row_group_stats() to MosaicWriter
- JNI: add 6 native functions for writer stats access
- FFI: add C API functions for writer stats (used by Python and C++)
- Java: MosaicWriter caches stats on close(), exposes via
  numRowGroups() and getRowGroupStatistics(rg)
- Python: MosaicWriter collects stats on close(), exposes via
  num_row_groups property and get_row_group_statistics(rg)
- Docs: updated all 4 language API docs (Rust, Java, Python, C++)