feat: target exact V2 versions in compaction

[Xuanwo]

Summary

• add an optional exact data storage version to compaction options
• carry the resolved target through distributed rewrite results
• allow binary copy only when every selected input matches the output target
• make TryBinaryCopy reencode mismatches and ForceBinaryCopy report target, actual version, and path

Stack

1. feat: add mixed data file version capability #8580 - capability reservation and commit gates
2. feat: validate mixed V2 data file versions #8581 - capability activation, 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

• homogeneous and mixed input compaction
• explicit and fallback targets
• TryBinaryCopy and ForceBinaryCopy mismatch behavior
• cargo check and clippy workspace gates