feat: add rc and rr transaction isolation

[KKould]

What problem does this PR solve?

KiteSQL did not expose configurable transaction isolation levels through DataBaseBuilder, and ordinary reads in RocksDB-backed transactions were not pinned to a stable statement-level or transaction-level snapshot.

That made it impossible to provide an explicit ReadCommitted / RepeatableRead contract at the database API layer, and it also left storage capabilities implicit instead of validated by the selected backend.

Issue link:

What is changed and how it works?

• Add TransactionIsolationLevel::{ReadCommitted, RepeatableRead} and plumb the selected level through DataBaseBuilder, Database, and storage transaction creation.
• Extend the storage abstraction with transaction_with_isolation, default_transaction_isolation, and validate_transaction_isolation so isolation semantics stay separate from storage-specific capabilities.
• Add statement-scope transaction hooks and use them to manage read snapshot lifetime during SQL execution.
• Implement RocksDB and optimistic RocksDB read visibility with explicit snapshots attached to ReadOptions:
  • ReadCommitted creates a fresh snapshot per statement.
  • RepeatableRead keeps one fixed snapshot for the whole transaction.
• Keep reads flowing through the underlying storage transaction object so transactions still observe their own writes.
• Declare LMDB support as RepeatableRead only and memory support as ReadCommitted only, with builder-time validation for unsupported level requests.
• Add user-facing transaction isolation documentation, expose it through crate docs, and link it from the README / feature docs.

Code changes

• Has Rust code change
• Has CI related scripts change

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Note for reviewer

The main review surface is:

1. DatabaseBuilder / Database plumbing for isolation-level selection and storage capability validation
2. RocksDB statement-scoped vs transaction-scoped snapshot handling for ReadCommitted and RepeatableRead
3. the transaction isolation support matrix and documentation entrypoints

Validation used for this branch:

• cargo check --lib
• cargo test --lib
• cargo test test_lmdb_rejects_read_committed_isolation --lib --features lmdb