Stage metadata for multi-operation Iceberg transactions

[osipovartem]

Summary

• Backport the transaction-staging behavior agreed in Preserve row lineage during v3 overwrites JanKaul/iceberg-rust#413 to the DF55 fork.
• Apply each operation to staged metadata so later operations see earlier snapshots, sequence numbers and v3 row IDs. Keep catalog requirements anchored to the committed base and validate staged requirements locally.
• Preserve the single-operation fast path without a metadata clone.
• Add v3 append+overwrite and stale-writer integration coverage.

Validation

• cargo +stable test --offline -p iceberg-rust --lib --test overwrite_test: 192 active library tests and 7 overwrite integration tests passed.
• cargo +stable clippy --offline -p iceberg-rust --lib --test overwrite_test: passed (pre-existing warnings in iceberg-sql-catalog).
• cargo +stable fmt --all -- --check and git diff --check: passed.
• Independent read-only review: APPROVED.