feat: support per-operation V2 write targets

[Xuanwo]

Summary

• honor explicit exact V2 targets for writes to existing datasets
• use the manifest storage version only as the operation fallback
• atomically activate mixed-version capability when a committed target differs
• preserve activation intent through retries, concurrency, and prewritten fragment commits

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

• explicit and fallback append targets
• sequential and concurrent mixed-version writers
• activation conflict retries and prewritten fragments
• cargo check and clippy workspace gates