P1: Split large API and test modules

[johnny-emp]

Summary

• split AedbInstance API methods into focused src/api modules for query, KV, backup/restore, reactive processors, and order book APIs
• split the monolithic lib_tests module into behavior-focused test modules while preserving test names
• extract query execution context, commit partition derivation helpers, and keyspace memory accounting helpers

Validation

• cargo check
• cargo check --tests
• cargo test --lib (455 passed, 7 ignored)
• cargo test commit::executor::tests
• cargo test storage::keyspace::tests