feat(raft): add nested transaction and CAS support

[slow2342]

Summary

Add compare-and-swap (CAS) transaction support with nested Txn capability. Switch RaftRequest/RaftResponse serialization from rkyv to bincode to enable recursive types. Fix several bugs discovered during implementation review.

Changes

• Cargo.toml — Add bincode = "1" dependency.
• proto/kv.proto — Add TxnRequest txn / TxnResponse txn to RequestOp / ResponseOp for nested Txn proto support.
• src/raft/mod.rs — Remove rkyv derives from all raft types, keep serde only. Add TxnRequest, TxnResponse structs. Add Request::Txn(Box<TxnRequest>) and Response::Txn(Box<TxnResponse>) variants.
• src/raft/raftrs_handle.rs — Switch RaftRequest/RaftResponse serialization from rkyv to bincode.
• src/raft/state_machine.rs — Switch deserialization to bincode. Extract evaluate_compare method. Implement nested Txn execution sharing parent's WriteBatch for atomicity. Fix TxnSnapshot::rollback() lease reconciliation order. Fix Get/Range/compare within Txn to read from value_cache for uncommitted values.
• src/api/kv.rs — Extract convert_compares, convert_response_ops, convert_kv helpers. convert_request_ops becomes fallible with MAX_TXN_DEPTH=1 depth limit. Add recursive check_txn_ops_permissions for nested Txn auth. Add raft::Response::Txn handling in response conversion.
• src/auth/role.rs — extract_txn_keys recursively handles nested Txn.

Testing

• 15 new CAS tests covering all compare targets (Version, Value, Create, Mod, Lease), all compare results (Equal, NotEqual, Greater, Less), multi-compare, failure branch, nonexistent key, Get/Delete in success branch.
• 2 new nested Txn tests: basic nested execution and uncommitted value read in nested compare.
• All 74 tests pass, clippy clean, fmt clean.

How to verify

cargo build
cargo test -p aether --lib
cargo clippy -- -D warnings
cargo fmt -- --check

Related issues

Refs #82