feat: propagate exact V2 versions across dataset operations

[Xuanwo]

Summary

• expose exact output targets on update and merge-insert builders
• propagate source and output versions independently through fast and slow rewrite paths
• add exact-version column writing and per-file schema metadata policy
• reject ambiguous metadata-only changes on mixed files when a rewrite is required

Stack

1. feat: add mixed data file version capability #8580 - capability and transaction plumbing
2. feat: validate mixed V2 data file versions #8581 - storage validation and exact read dispatch
3. feat: support per-operation V2 write targets #8582 - per-operation write targets
4. feat: propagate exact V2 versions across dataset operations #8583 - dataset operation propagation
5. feat: target exact V2 versions in compaction #8584 - compaction targets and binary copy policy
6. feat: expose exact V2 versions in bindings #8585 - bindings, documentation, and rollout contract

Testing

• update and merge-insert explicit target tests
• full, partial, and nested metadata operation coverage
• cargo fmt, workspace check, and workspace clippy