feat: expose exact V2 versions in bindings - #8585

Summary

• expose the same data_storage_version selector for Python and Java update, merge-insert, and compaction
• preserve compaction targets across Python pickle and Java serialization and JNI round trips
• document fallback semantics, compatibility states, error categories, and the rollout gate
• add binding-level exact-version and distributed compaction coverage

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

• Python exact-version update, merge-insert, and distributed compaction tests
• uv run make lint
• Java UpdateTest, MergeInsertTest, and CompactionTest
• Java JNI Rust tests
• cargo fmt --all -- --check
• cargo check --workspace --tests --benches
• cargo clippy --all --tests --benches -- -D warnings