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 layer 2 of GitHub stack apache#8 and depends on #1.

• 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.

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 stack layer)
• Full stack: make check-fmt
• Full stack: make check-clippy
• git diff --check

AI Disclosure

This PR was prepared with assistance from Codex.