Prevent V2 manifest reuse after unpartitioned spec evolution

[osipovartem]

Summary

• Apply writer-compatibility selection to V2 unpartitioned append and COW paths, matching the existing partitioned path.
• Preserve old partitioned manifests after the table switches to an unpartitioned spec. Route affected old manifests to the explicit cross-spec COW rejection rather than rewriting them under the wrong spec.

Verification

• Regression forces a manifest split with eight new files, verifies all ten file paths, then checks compatible COW selection and explicit rejection of an old-spec rewrite.
• Fork lib tests: 188 passed, 2061 ignored.
• Clippy +1.94 -p iceberg-rust --all-targets -- -D warnings: passed.
• Independent read-only review: APPROVED.