refactor: split integration tests into focused modules

[krlvi]

Summary

• Break the monolithic integration.rs into 8 purpose-driven test modules (crud, value_types, handle, serialize, exchange, merge, edge_cases, session)
• Extract shared test utilities into helpers/mod.rs
• Each module is self-contained and documents a specific aspect of the API

Test plan

• cargo test --workspace passes (all 138+ tests)
• No test coverage lost — same tests, better organization

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