feat(parquet): omit unknown fields from writes

[manuzhang]

Which issue does this PR close?

No issue is currently linked.

What changes are included in this PR?

This is the Parquet-write follow-up to #2773 and is intentionally opened as a draft.

• Builds the physical Parquet Arrow schema by omitting Unknown fields.
• Rejects containers that would have no valid physical Parquet representation.
• Projects incoming record batches to the physical schema before writing while preserving nested container structure and row counts.
• Updates NaN statistics traversal to work against the projected physical batch.

Until #2773 merges, this draft temporarily includes the Unknown logical-type commit in its cumulative diff. After #2773 merges, the branch will be rebased onto main so this PR contains only the Parquet-write commit.

Are these changes tested?

• Added schema and batch-projection regressions for top-level and nested Unknown fields, invalid empty containers, and name-based matching.
• cargo test -p iceberg --lib (1,594 passed at this follow-up layer)
• make check-fmt
• make check-clippy
• git diff --check

AI Disclosure

This PR was prepared with assistance from Codex.