feat(parquet): reconstruct unknown fields on reads

[manuzhang]

Which issue does this PR close?

No issue is currently linked.

What changes are included in this PR?

This is the Parquet-read follow-up to #2773 and #3072 and is intentionally opened as a draft.

• Excludes non-physical Unknown fields from Parquet projection masks and reconstructs them as Arrow null arrays.
• Preserves row counts for empty and Unknown-only projections.
• Makes embedded field IDs and name mappings authoritative, with guarded positional fallback for files without either.
• Reconstructs missing nested fields and materializes primitive and container initial defaults.
• Includes the requested top-level struct/list/map default fix and regression rather than discarding non-primitive defaults.

Until #2773 and #3072 merge, this draft temporarily includes their commits in its cumulative diff. After both predecessors merge, the branch will be rebased onto main so this PR contains only the Parquet-read commit.

Are these changes tested?

• Added read-path regressions for Unknown-only projections, filters, schema evolution, missing/deleted IDs, nested fallback, and primitive/nested/top-level defaults.
• cargo test -p iceberg --lib (1,610 passed)
• make check-fmt
• make check-clippy
• git diff --check

AI Disclosure

This PR was prepared with assistance from Codex.