Split core block persistence modules

[yvette-carlisle]

Summary

• split core_blocks/persistence.rs into a small facade plus private blocks, attachments, readback, and audit modules
• keep the existing core_blocks::persistence service call surface intact through private re-exports
• preserve SQL strings, transaction flow, snapshots, and DTO/type ownership while replacing the 424-line persistence file with focused modules

Validation

• cargo make fmt-rust
• cargo make check-rust
• cargo make lint-vstyle
• cargo make lint-rust
• cargo make test-rust
• cargo make check
• read-only skeptic review: no blockers