feat(writer): add PositionDeleteFileWriter

[laskoviymishka]

Which issue does this PR close?

Closes #340.

What changes are included in this PR?

This adds PositionDeleteFileWriter under writer/base_writer, next to the data
and equality-delete writers. We already had a writer for equality deletes but
nothing that writes position delete files, so I modelled this one on
equality_delete_writer.rs.

It writes the two required columns, file_path (string, field id 2147483546) and
pos (long, 2147483545), and sets the content type to PositionDeletes on close.
position_delete_schema() / position_delete_arrow_schema() hand back the
canonical schema, built from the field definitions already in metadata_columns
so the reserved ids aren't duplicated here.

write() checks the batch is exactly those two required, non-null, correctly
typed columns before handing it to the parquet writer, so a bad batch fails with a
clear message instead of a confusing error further down. close() carries the
partition key onto the data file and leaves sort_order_id null.

One thing worth calling out: this writer doesn't sort its input. Position deletes
have to be sorted by file_path then pos, and for now that's on the caller (the
write() docs say so). I'd rather land the plain writer first and put sorting on
top of it, so a sorting writer, referenced_data_file, and a higher-level
DeltaWriter (#2218) are follow-ups instead of being crammed in here.

Are these changes tested?

Yes, 12 unit tests: the schema shape, a parquet round trip for single and multiple
writes (write it, read it back, compare), partition and spec-id propagation on
close, and the rejection cases: wrong column count, wrong or missing field ids on
either column, wrong types (including LargeUtf8, which is what a lot of Arrow
producers give you), nullable columns, and writing after close. cargo test -p iceberg, clippy, and fmt are all clean.

AI assistance disclosure

Drafted with help from Claude Code, then reviewed and tested by me before opening.

[anoopj]

I took a pass. Overall, the implementation looks mostly good to me. Some feedback:

[laskoviymishka]

Thanks for the review! Pushed a follow-up commit addressing all four:

• Added a note to the module docs that position delete files are a v2 construct — v3 replaces them with deletion vectors and forbids new position delete files, so callers must not route v3 writes here, and format-version gating stays at the transaction/commit layer rather than this base writer.
• Made position_delete_arrow_schema crate-internal. Since only the tests consume it today, I gated it (and its backing static) under #[cfg(test)] to avoid a dead-code warning; I'll widen it to pub(crate) when a higher-level writer actually needs the Arrow form. position_delete_schema stays public.
• Added a test that reads the written file's Parquet schema and asserts the two reserved column field ids (2147483546 / 2147483545) survive the write.
• Added a test for the field_id parse-error branch (metadata present but not an integer).

Also merged latest main in. Let me know if you'd prefer position_delete_arrow_schema kept as pub(crate) regardless — happy to switch.

[anoopj]

LGTM. There is a CI failure though:

  error: `unparseable` should be `unparsable`
      ╭▸ ./crates/iceberg/src/writer/base_writer/position_delete_writer.rs:593:50
      │
  593 │     async fn test_position_delete_writer_rejects_unparseable_field_id() -> Result<()> {

It's a bit weird that the checker thinks unparseable is a typo. (both are correct forms)

[laskoviymishka]

@CTTY / @blackmwk PTAL