feat: implement SSI predicate locking

[danpasecinic]

This pull request introduces Serializable Snapshot Isolation (SSI) to the MVCC engine, enabling true serializable transactions and improving transaction isolation guarantees. It adds a new SSI manager, updates transaction logic to support various serializable modes, and introduces comprehensive error handling and testing for serializability conflicts.

Serializable Snapshot Isolation (SSI) Integration:

• Added the SSIManager and related types to the MVCC engine (MvccEngine), with new logic for tracking reads and writes, validating serializability at commit, and handling aborts and cleanup. (crates/crabstash-txn/src/mvcc.rs, crates/crabstash-lock/src/lib.rs) [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

• Extended the IsolationLevel enum to support SerializableReadOnly and SerializableDeferrable modes, with helper methods for checking isolation properties. (crates/crabstash-txn/src/transaction.rs)

Error Handling Improvements:

• Added new error variants for serializability violations (WriteSkew, PhantomRead, SerializableConflict) and mapped SSI conflict errors to these variants. (crates/crabstash-common/src/error.rs, crates/crabstash-txn/src/mvcc.rs) [1] [2] [3]

Testing and Validation:

• Introduced comprehensive tests for serializable isolation, including detection of write skew, phantom reads, and proper cleanup of SSI state on abort. (crates/crabstash-txn/src/mvcc.rs)

Dependency and API Updates:

• Added rapidhash as a dependency for workspace crates, likely used by the SSI manager. (Cargo.toml, crates/crabstash-lock/Cargo.toml) [1] [2]
• Updated the API to pass the new isolation levels through the public interface, simplifying how isolation is handled in the top-level Db type. (crates/crabstash/src/lib.rs) [1] [2]

Development Tooling:

• Added tempfile as a dev-dependency to facilitate testing. (crates/crabstash-txn/Cargo.toml)